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Edexcel A2 chemistry
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NT Exampro 1
1. Propanoic acid is a weak acid; explain the term weak.
....................................................................................................................................
.................................................................................................................................... (1)
2. When ammonium salts are dissolved in water, the following reaction occurs.
NH4+ (aq) + H2O(1) NH3(aq) + H3O+ (aq)
(a) Identify the acid/base conjugate pairs in this reaction by writing appropriate symbols
under each of the species in the equation above.
(b) Write an expression for the dissociation constant, Ka, for NH4+ (aq).
(c) Calculate the pH of a solution of ammonium chloride of concentration 0.100 mol dm–3
at 298 K, the Ka value for NH4+ being 5.62 × 10–10 mol dm–3 at this temperature.
(Total 5 marks)
3. There is no suitable indicator for the titration of ethanoic acid with ammonia.
Why is this?
……..………...................……….......................................................................................
……..………...................……….......................................................................................
……..………...................………....................................................................................... (2)
4. (a) Concentrated sulphuric acid reacts with sodium chloride as follows:
H2SO4 + Cl– HCl + HSO4(i) Identify the conjugate
acid/base pairs in this reaction by writing suitable identifying symbols under the
equation. (2)
NT Exampro 2
(ii) What would be the observable result of this reaction?
.......................................................................................................................... (1)
(iii) Explain why this reaction goes almost completely to the right despite the fact that
both hydrochloric and sulphuric acids are strong.
..........................................................................................................................
..........................................................................................................................
.......................................................................................................................... (3)
(b) When concentrated sulphuric acid reacts with solid sodium iodide, hydrogen sulphide,
sodium hydrogensulphate and water are formed together with one other product.
(i) Identify this product and state how you would recognise it.
.......................................................................................................................... (2)
(ii) Write an ionic half equation to show the conversion of sulphuric acid to
hydrogen sulphide.
..........................................................................................................................
.......................................................................................................................... (2)
(iii) Hence write the full ionic equation for the reaction between concentrated sulphuric
acid and sodium iodide.
..........................................................................................................................
..........................................................................................................................
.......................................................................................................................... (2)
(iv) What is the function of the sulphuric acid in this reaction?
.......................................................................................................................... (1)
NT Exampro 3
(c) Explain concisely why the type of reaction occurring in (b) does not occur with
sodium chloride.
....................................................................................................................................
....................................................................................................................................
.................................................................................................................................... (2)
(Total 15 marks)
5. Acid-base indicators are weak acids, which can be represented as HIn. In aqueous solution the
following equilibrium is set up:
HIn(aq) H+(aq) + In–(aq)
HIn and In– have different colours. The dissociation constant for the indicator equilibrium is
called in.
pH is related to pKin by the equation
pH = pKin + log]HIn[
]In[−
(a) (i) For bromophenol blue Kin = 1 × 10–4 mol dm–3. Calculate the pH of a solution in
which [In–] = [HIn] for this indicator.
(2)
(ii) The colour of In– is seen when [In–] is roughly ten times [Hln], and the colour of
HIn when it is in tenfold excess over In–. Use this information to show the
approximate range of pH over which indicator changes can be detected visually. (2)
(b) The selection of a correct indicator for a reaction depends on the nature of the reaction and
the concentrations of the substances used. The neutralisation curves for the addition of
sodium hydroxide solution to 100 cm3 of hydrochloric acid of the same concentration are
shown for two different concentrations of acid and alkali.
NT Exampro 4
(i) It is often said that the titration of a strong acid with a strong alkali can be
performed with any indicator. In the light of the table and titration curves given,
comment on this statement. (2)
Indicator
Methyl orange
Bromophenol blue
Methyl red
Bromothymol blue
Phenolphthalein
Thymolphthalein
3.7
4.0
5.1
7.0
9.3
9.7
pK In14
12
10
8
6
4
2
0
pH
0 50 100 150 200
1.00 mol dm
0.01 mol dm
0.01 mol dm
1.00 mol dm
–3
–3
–3
–3
3volume of alkali added / cm
Titration of 100 cm3 of hydrochloric acid with
sodium hydroxide of the same concentration
(ii) From a consideration of the curve below suggest a practical difficulty that could
arise if a titration were to be attempted using dilute solutions of ethanoic acid and
ammonia.
14
12
10
8
6
4
2
0
pH
0 25 50
volume of alkali added / cm 3
Titration of 25 cm3 of 0.1 mol dm–3 ethanoic acid
with 0.1 mol dm–3 ammonia (2)
NT Exampro 5
(iii) In the titration of acidic solutions with sodium hydroxide solution using
phenolphthalein as indicator, the presence of carbon dioxide in the air can cause
practical difficulties of which the analyst needs to be aware. What is this difficulty
and why does it occur? (2)
(c) The titration curve for the addition of sodium hydroxide solution to ethanoic acid
is shown.
14
12
10
8
6
4
2
0
pH
0 25 503volume of 0.1 mol dm sodium hydroxide solution / cm3–3
Titration of 25 cm3 of 0.1 mol dm–3 ethanoic acid
with 0.1 mol dm–3 sodium hydroxide
(i) Explain why the pH of the solution when the acid has been half-neutralised is equal
to pKa for the acid.
(ii) Explain why at this pH the mixture acts as a buffer and why it is most effective at
this pH (7)
(d) Methyl orange is an azo dye, usually used as its sodium salt. It can be made by the series
of reactions shown in the flow-scheme below.
CH CH
CH CH
N N +
NH
N NO S
O S O S
O SN N
3 3
+
N N+
3
3 3
3–
– –
–in NaOH(aq)
NaNO , HCl0 – 5 ºC
3
22
3
(i) Explain why the temperature for the first step should be above 0 °C and below 5 °C. (2)
(ii) Primary amines and ammonia react with halogenoalkanes in a similar manner.
Suggest how the compound C6H5N(CH3)2 could be obtained, and give an equation
to represent the reaction for its formation. (2)
NT Exampro 6
(iii) Methyl orange is soluble in hot water, fairly soluble in cold water, and even less so
in cold aqueous sodium chloride. Suggest in outline how a sample of impure
methyl orange could be purified so as to maximize the yield. A detailed
experimental account is not required. (4)
(Total 25 marks)
6. (a) Define the following terms.
(i) pH ..................................................................................................................... (1)
(ii) Kw ......................................................................................................................
(1)
(b) Explain the meaning of the term strong, as applied to an acid or a base.
………………...……………….……………………………………………………..
………………...……………….…………………………………………………….. (1)
(c) Calculate the pH of the following solutions.
(i) HCl(aq) of concentration 0.200 mol dm–3.
(1)
(ii) NaOH (aq) of concentration 0.800 mol dm–3 (Kw = – 1.00 × 10–14 mol2 dm–6).
(2)
(d) HA is a weak acid with a dissociation constant Ka = 5.62 × 10–5 mol dm–3.
(i) Write an expression for the dissociation constant, Ka, of HA.
(1)
NT Exampro 7
(ii) Calculate the pH of a 0.400 mol dm–3 solution of HA.
(3)
(e) A buffer solution contains HA(aq) at a concentration of 0.300 mol dm–3, and its sodium
salt, NaA, at a concentration of 0.600 mol dm–3. Calculate the pH of this buffer solution.
(3)
(Total 13 marks)
7. Propanoic acid is a weak acid which dissociates according to
CH3CH2COOH(aq) + H2O(l) CH3CH2COO–(aq) + H3O+(aq)
(a) (i) Indicate, in the space provided below the equation, the two acid/base conjugate
pairs. (2)
(ii) Write the expression for the acid dissociation constant, Ka, for propanoic acid.
(1)
NT Exampro 8
(iii) Calculate the pH of a 0.100 mol dm–3 solution of propanoic acid, for which
Ka = 1.3 × 10–5 mol dm–3.
(3)
(iv) Calculate the concentration of hydroxide ions, OH–, in this same solution of
propanoic acid. Kw = 1.00 × 10–14 mol2 dm–6 at the temperature of the solution.
(3)
(b) If sodium propanoate is dissolved in water, the pH of the resulting solution is not 7, but is
near to 8. By writing the equation for the reaction occurring suggest why this is so.
................................................................................................................................
................................................................................................................................
................................................................................................................................ (2)
(c) A mixture of sodium propanoate and propanoic acid behaves as a buffer solution.
(i) What is meant by a buffer solution?
......................................................................................................................
......................................................................................................................
...................................................................................................................... (2)
NT Exampro 9
(ii) Calculate the pH of a buffer solution that is made by mixing equal volumes of
0.0500 mol dm–3 propanoic acid and 0.100 mol dm–3 sodium propanoate.
(3)
(Total 16 marks)
8. (a) (i) Calculate the concentration, in mol dm–3, of a solution of hydrochloric acid, HCl,
which has a pH of 1.13.
(1)
(ii) Calculate the concentration, in mol dm–3 , of a solution of chloric(l) acid, HOCl,
which has a pH of 4.23.
Chloric(l) acid is a weak acid with Ka = 3.72 × 10–8 mol dm–3.
(4)
(b) The pH of 0.100 mol dm–3 sulphuric acid is 0.98.
(i) Calculate the concentration of hydrogen ions, H+, in this solution.
(1)
NT Exampro 10
(ii) Write equations to show the two successive ionisations of sulphuric acid, H2SO4, in
water.
......................................................................................................................
...................................................................................................................... (2)
(iii) Suggest why the concentration of hydrogen ions is not 0.20 mol dm–3 in 0.100 mol
dm–3 sulphuric acid.
......................................................................................................................
...................................................................................................................... (1)
(c) Many industrial organic reactions produce hydrogen chloride as an additional product.
This can be oxidised to chlorine by the Deacon process:
4HCl(g) + O2(g) 2Cl2(g) + 2H2O(g) ∆H = –115 kJ mol–1.
0.800 mol of hydrogen chloride was mixed with 0.200 mol of oxygen in a vessel of
volume 10.0 dm3 in the presence of a copper(I) chloride catalyst at 400 ºC. At
equilibrium it was found that the mixture contained 0.200 mol of hydrogen chloride.
(i) Write an expression for the equilibrium constant Kc.
(1)
(ii) Calculate the value of Kc at 400 ºC.
(4)
NT Exampro 11
(d) State and explain the effect, if any, on the position of equilibrium in (c) of:
(i) decreasing the temperature;
......................................................................................................................
......................................................................................................................
...................................................................................................................... (2)
(ii) decreasing the volume;
......................................................................................................................
......................................................................................................................
...................................................................................................................... (2)
(iii) removing the catalyst.
......................................................................................................................
......................................................................................................................
...................................................................................................................... (2)
(Total 20 marks)
9. The values of Ka for a series of organic acids at 25 °C are shown in the table below:
Acid Formula Ka/mol dm–3
ethanoic acid CH3COOH 1.75 × 10–5
chloroethanoic acid CH2(Cl)COOH 1.38 × 10–3
bromoethanoic acid CH2(Br)COOH 1.26 × 10–3
iodoethanoic acid CH2(I)COOH 6.76 × 10–4
NT Exampro 12
(a) (i) Give the expression for Ka for ethanoic acid.
(1)
(ii) Calculate the pH of 0.100 mol dm–3 ethanoic acid.
(3)
(iii) Calculate the pH of a mixture which is 0.100 mol dm–3 with respect to ethanoic
acid and 0.125 mol dm–3 with respect to sodium ethanoate.
(iv) What property is shown by such a mixture?
…..……………………………………………………………………………..
…..……………………………………………………………………………..
…..…………………………………………………………………………….. (2)
(b) (i) By consideration of bond polarity, suggest why 1-chloroethanoic acid is a stronger
acid than ethanoic acid.
…..……………………………………………………………………………..
…..……………………………………………………………………………..
…..…………………………………………………………………………….. (2)
(ii) Suggest a reason for the trend Ka values shown in the table above.
…..……………………………………………………………………………..
…..…………………………………………………………………………….. (1)
NT Exampro 13
(iii) Suggest, with an explanation, whether 1 -fluoroethanoic acid is likely to be a
stronger or a weaker acid than 1-chloroethanoic acid.
…..……………………………………………………………………………..
…..……………………………………………………………………………..
…..…………………………………………………………………………….. (2)
(c) Ethanoyl chloride, CH3COCl, reacts violently with water, whereas chloroethanoic acid
reacts much more slowly in order to substitute the chlorine atom. Suggest why this is so.
…..………………………….………………………………………………………..
…..………………………….………………………………………………………..
…..………………………….……………………………………………………….. (2)
(Total 15 marks)
10. (a) Define
(i) pH : …………………………………………………………………………… (1)
(ii) Kw : ……………………………………………………………………………
(1)
(b) Calculate the pH of 0.100 mol dm–3 sodium hydroxide solution at 25 ºC. The value of Kw
at this temperature is 1.0 × 10–14 mol2 dm–6. (2)
(c) 100 cm3 of 0.100 mol dm–3 sodium hydroxide solution was added to 100 cm3 of 0.200 mol
dm–3 ethanoic acid.
(i) Find the concentration of ethanoic acid in the mixture. (2)
(ii) Calculate the concentration of sodium ethanoate in the mixture. (2)
(iii) Calculate the pH of the mixture at 25 ºC; Ka for ethanoic acid at this temperature is
1.80 × 10–5 mol dm –3. (2)
(iv) State and explain what happens if a small amount of hydrochloric acid is added to
this mixture.
…………………………………………………………………………………
…………………………………………………………………………………
………………………………………………………………………………… (3)
(d) When the solutions in part (c) are mixed, 560 J of heat energy is evolved. If hydrochloric
acid of the same concentration is used instead of ethanoic acid, 580 J of heat energy is
NT Exampro 14
evolved. Suggest reasons for the difference.
…………………………………………………………………………..……………
…………………………………………………………………………..……………
…………………………………………………………………………..……………
…………………………………………………………………………..…………… (3)
(Total 16 marks)
11. Citric acid is used in foodstuffs as an antioxidant and, together with its sodium salt, as an acidity
regulator. It occurs naturally in fruit juices.
A formula of citric acid is
CH CO H
HO C CO H
CH CO H•
2
2
2
2
2
(a) (i) Assuming citric acid behaves in aqueous solution as a monoprotic acid:
RCO2H + H2O –2RCO + H3O+
write an expression for Ka for this acid.
(1)
(ii) Calculate the pH of lemon juice which contains citric acid at a concentration of
0.200 mol dm–3. (Ka for citric acid = 7.4 × 10–4 mol dm–3).
(3)
(b) The use of citric acid together with its salt, sodium citrate, as an acidity regulator depends
on the ability of this mixture to act as a buffer.
(i) What is the function of a buffer solution?
…………………………………………………………………………………
………………………………………………………………………………… (2)
NT Exampro 15
(ii) Describe how the mixture of citric acid and sodium citrate achieves this buffering
action. Give equations for the TWO reactions you describe.
…………………………………………………………………………………
…………………………………………………………………………………
…………………………………………………………………………………
………………………………………………………………………………… (3)
(iii) Calculate the pH of a buffer solution containing 0.200 mol dm–3 of citric acid and
0.400 mol dm–3 of sodium citrate. (2)
(c) Citric acid forms a liquid ester which has the structural formula
CH COOC H
HO C COOC H
CH COOC H
2
2
2 5
2 5
2 5
(i) Describe a test you could use to show that the ester contains an —OH group.
…………………………………………………………………………………
…………………………………………………………………………………
………………………………………………………………………………… (2)
(ii) What reagent would you use to hydrolyse the ester?
…………………………………………………………………………………
………………………………………………………………………………… (1)
(iii) Treatment of the products of the reaction in (c)(ii) leads to the production of a pure
sample of citric acid. How would you show the presence of the —CO2H group in
the citric acid other than by the use of an indicator?
…………………………………………………………………………………
………………………………………………………………………………… (2)
(Total 16 marks)
NT Exampro 16
12. Acids differ in the number of hydrogen ions that can be liberated from one molecule of the
undissociated acid. Hydrochloric acid is a strong monobasic, or monoprotic acid, liberating one
hydrogen ion per molecule. Sulphuric acid is a dibasic, or diprotic acid, its ionisation in aqueous
solution being:
I H2SO4(1) + H2O(1) H3O+ (aq) + –4HSO (aq) Ka = very large
II –4HSO (aq) + H2O(1) H3O+ (aq) + –2
4SO (aq) Ka = 0.01 mol dm–3
Ka values are quoted for 25 ºC.
(a) (i) State the hydrogen ion concentration in 0.01 mol dm–3 hydrochloric acid, which is a
strong acid, and hence find the pH of this solution. (1)
(ii) State the hydrogen ion concentration of 0.1 mol dm–3 sulphuric acid arising from the
first stage of ionisation, I. (1)
(iii) A solution of sodium hydrogensulphate, NaHSO4, of concentration 0.1 mol dm–3,
which ionises according to equation II above has a pH of 1.57. Find the hydrogen
ion concentration in this solution, and hence state what you would expect the
hydrogen ion concentration in 0.1 mol dm–3 sulphuric acid to be. (2)
(iv) In fact the pH of 0.1 mol dm–3 sulphuric acid is about 0.98. This indicates a
hydrogen ion concentration of 0.15 mol dm–3. By considering the reactions I and II
in the presence of one anotherexplain why this is so. You are not expected to
perform any further calculations.
….……………………..………………………………………………………
….……………………..………………………………………………………
….……………………..……………………………………………………… (2)
(v) If Ka for ionisation II has a value of 0.02 mol dm–3 at 80 °C, state with reasons,
whether the ionisation is endothermic or exothermic.
….……………………..………………………………………………………
….……………………..……………………………………………………… (2)
(vi) Explain the effect such an increase in temperature would have on the pH of
this solution.
….……………………..……………………………………………………… (1)
NT Exampro 17
(b) Pure sulphuric acid is a liquid, with a boiling temperature of 338 °C, which mixes with
water in all proportions. Hydrogen chloride is a gas with a boiling temperature of –85 °C
and is extremely soluble in water.
(i) Suggest reasons, in terms of the structure and bonding of H2SO4 and HCl for the
large difference between their boiling temperatures.
….……………………..………………………………………………………
….……………………..……………………………………………………… (2)
(ii) Suggest why both are very soluble in water.
….……………………..……………………………………………………… (1)
(iii) Magnesium sulphate and barium sulphate differ considerably in their solubility in
water; use the following data to suggest why this is so.
MgSO4 BaSO4
Solubility/mol dm–3 1.83 9.43 × 10–6
Hydration energy of cation /kJ mol–1 –1920 –1360
….……………………..………………………………………………………
….……………………..………………………………………………………
….……………………..………………………………………………………
….……………………..……………………………………………………… (3)
(Total 15 marks)
13. A buffer solution of pH =3.87 contains 7.40 g dm–3 of propanoic acid together with a quantity of
sodium propanoate. Ka for propanoic acid = 1.35 × 10–5 mol dm–3 at 298 K.
(i) Explain what a buffer solution is and how this particular solution achieves its buffer
function. (8)
(ii) Calculate the concentration in g dm–3 of sodium propanoate, C2H5CO2Na, in the
solution, stating any assumptions made. (6)
(iii) If the sodium propanoate were to be replaced by anhydrous magnesium propanoate,
calculate the concentration of magnesium propanoate in g dm–3, required to give a buffer
of the same pH. (2)
(Total 16 marks)
14. Concentrated sulphuric acid reacts with sodium chloride as follows:
NT Exampro 18
H2SO4 + Cl– HCl + HSO4–
(i) Identify the conjugate acid/base pairs in this reaction by writing suitable identifying
symbols under the equation. (2)
(ii) What would be the observable result of this reaction?
...........………............................................................................................................. (1)
(iii) Explain why this reaction goes almost completely to the right despite the fact that both
hydrochloric and sulphuric acids are strong.
...........……….............................................................................................................
...........……….............................................................................................................
...........………............................................................................................................. (3)
(Total 6 marks)
15. (a) Propanoic acid dissociates in aqueous solution according to
CH3CH2CO2H(aq) + H2O(1) CH3CH2CO2–(aq) + H3O+(aq)
(i) Write the expression for Ka for propanoic acid.
(1)
(ii) Given that Ka for this reaction is 1.35 × 10–5 mol dm–3 at 298 K, calculate the pH of
a 0.100 mol dm–3 solution of propanoic acid at this temperature.
(3)
(iii) If the solution referred to in (ii) were to be diluted tenfold, it might be expected that
the pH would rise by one unit. In fact it rises by less than this. Explain.
..........................................................................................................................
..........................................................................................................................
.......................................................................................................................... (3)
NT Exampro 19
(b) During a titration NaOH solution of concentration 0.100 mol dm–3 was added to 30.0 cm3
of 0.100 mol dm–3 propanoic acid.
(i) What property is shown by the reaction mixture when 15.0cm3 of NaOH has been
added to the propanoic acid?
.......................................................................................................................... (1)
(ii) Calculate the pH of the reaction mixture at this stage.
Propanoic acid has Ka = 1.35 × 10–5 mol dm–3, pKa = 4.87.
(3)
(iii) What criteria must be considered in the selection of an indicator for this titration?
..........................................................................................................................
.......................................................................................................................... (2)
(c) When hydrochloric or nitric acid is reacted with sodium hydroxide solution, the enthalpy
change is –57.1 kJ mol–1. If propanoic acid is used instead of the other acids, the
enthalpy change is less negative.
(i) Suggest why this is so.
..........................................................................................................................
.......................................................................................................................... (2)
(ii) What would be the effect on the value of ∆H for the reaction of using ammonia
solution instead of sodium hydroxide with propanoic acid?
.......................................................................................................................... (1)
(Total 16 marks)
16. (a) When ethanoic acid is dissolved in water, the following equilibrium is established:
CH3CO2H + H2O CH3CO2– + H3O+ I
When hydrogen chloride dissolves in ethanoic acid, the equilibrium established is:
CH3CO2H +HCI CH3CO2H2+ + Cl– II
NT Exampro 20
Comment on the role of the ethanoic acid in:
(i) equilibrium I; .................................................................................................
.......................................................................................................................... (1)
(ii) equilibrium II; ................................................................................................
.......................................................................................................................... (1)
(b) What is the relationship between the species CH3CO2H2+ and CH3CO2H?
.................................................................................................................................... (1)
(c) The value of Ka for ethanoic acid at 298 K is 1.74 × 10–5 mol dm–3 and for methanoic
acid, HCO2H, it is 1.60 × 10–4 mol dm–3 at the same temperature.
(i) Write an expression for Ka for CH3CO2H.
(1)
(ii) Hence calculate the pH of a 0.100 mol dm–3 solution of CH3CO2H at 298 K.
(2)
(d) The pH of a 0.050 mol dm–3 solution of HCO2H is 2.55.
Using this, together with the data in (c) and your answer to (c)(ii):
(i) state which of the two acids is the stronger;
.......................................................................................................................... (1)
(ii) comment on the relative pH values of the two acids.
..........................................................................................................................
.......................................................................................................................... (3)
(e) (i) Sketch with reasonable accuracy, on the axes below, how the pH changes during
the titration of 20.0 cm3 of a 0.100 mol dm–3 solution of methanoic acid with 0.050
mol dm–3 sodium hydroxide solution.
NT Exampro 21
Volume of NaOH solution/cm
12
10
8
6
4
2
00 40 45 505 10 15 20 25 30 35 55 60
pH
3
(3)
(ii) Select using the data below a suitable indicator for this titration. Give a brief
reason for your choice based on the curve drawn in (e)(i).
Indicator pH Range
Bromocresol green 3.5 – 5.4
Bromothymol blue 6.0 – 7.6
Phenol red 6.8 – 8.4
..........................................................................................................................
.......................................................................................................................... (2)
(Total 15 marks)
NT Exampro 22
17. (a) Sketch on the axes below the pH curve for the titration of 25 cm3 of 0.1 mol dm–3 HCl
with 0.1 mol dm–3 ammonia.
14
7
0
pH
0 10 20 30 40 50 0.1 mol dm NH /cm33–3
(b) What indicator would you use for this titration?
……..………...................………............................................................................... (Total 3 marks)
18. Propanoic acid dissociates in aqueous solution according to
CH3CH2CO2H(aq) + H2O(1) CH3CH2CO2–(aq) + H3O+(aq)
(a) Write the expression for Ka for propanoic acid.
(1)
(b) Given that Ka for this reaction is 1.35 × 10–5 mol dm–3 at 298 K, calculate the pH of a
0.100 mol dm–3 solution of propanoic acid at this temperature.
(3)
NT Exampro 23
(c) If the solution referred to in (ii) were to be diluted tenfold, it might be expected that the
pH would rise by one unit. In fact it rises by less than this. Explain.
.....................………...................................................................................................
.....................………...................................................................................................
.....................………................................................................................................... (3)
(Total 7 marks)
19. (i) Give an equation for the dissociation of propanoic acid and hence an expression for its
dissociation constant, Ka.
..........................................................................................................................
(ii) At 25°C Ka for propanoic acid is 1.30 × 10–5 mol dm–3. Find the pH of a solution of
propanoic acid of concentration 0.0100 mol dm–3. State any assumptions you make.
(iii) Increasing the temperature of the propanoic acid solution causes the pH to decrease.
What does this tell you about the enthalpy of dissociation?
Justify your answer.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
.......................................................................................................................... (9)
20. (a) Ammonia is a base. What is meant by the term base?
.....................………....................................................................................................
(b) What feature of the ammonia molecule enables it to react as a base?
.………........................................................................................................................
NT Exampro 24
(c) Given that Kb = [OH NH
NH
4
3
− +][ ]
[ ] calculate the pH of a 0.100 mol dm–3 solution
of ammonia.
The values of Kb and Kw are 1.80 × 10–5 mol dm–3 and 1.00 × 10–14 mol2 dm–6
respectively.
(Total 5 marks)
21. (a) (i) Ammonia is a base. What is meant by the term base?
..........................................................................................................................
(ii) What feature of the ammonia molecule enables it to react as a base?
..........................................................................................................................
(iii) Given that Kb = [OH NH
NH
4
3
− +][ ]
[ ] calculate the pH of a 0.100 mol dm–3 solution
of ammonia.
The values of Kb and Kw are 1.80 × 10–5 mol dm–3 and 1.00 × 10–14 mol2 dm–6
respectively.
(5)
NT Exampro 25
(b) (i) Sketch on the axes below the pH curve for the titration of 25 cm3 of 0.1 mol dm–3
HCl with 0.1 mol dm–3 ammonia.
14
7
0
pH
0 10 20 30 40 50 0.1 mol dm NH /cm33–3
(ii) What indicator would you use for this titration?
.......................................................................................................................... (3)
(c) There is no suitable indicator for the titration of ethanoic acid with ammonia.
Why is this?
..........................................................................................................................
..........................................................................................................................
.......................................................................................................................... (2)
(d) Suggest the conditions under which ammonia could be used to convert CH3CHClCO2H to
CH3CH(NH2)CO2H.
.................................................................................................................................... (2)
(e) Explain how a solution of 2-aminopropanoic acid, CH3CH(NH2)CO2H, can act
as a buffer.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
.................................................................................................................................... (3)
(Total 15 marks)
NT Exampro 26
22. (a) (i) Define pH. .........................................................................................................
(ii) Define Kw, the ionic product of water.
.......................................................................................................................... (2)
(b) Calculate the pH of the following solutions.
(The ionic product of water, Kw, may be taken as 1.00 × 10–14 mol2 dm–6.)
(i) A solution of sulphuric acid having a concentration of 0.100 mol dm–3.
(ii) A solution of sodium hydroxide having a concentration of 0.0500 mol dm–3.
(3)
(c) (i) What is the principal property of a buffer solution?
..........................................................................................................................
..........................................................................................................................
(ii) The dissociation constant for ethanoic acid is 1.80 × 10–5 mol dm–3. Calculate the
pH of a buffer solution which has a concentration of 0.0150 mol dm–3 with respect
to ethanoic acid and 0.0550 mol dm–3 with respect to sodium ethanoate.
(5)
(d) Phosphoric(V) acid, H3PO4 , is a tribasic acid.
Write the formulae of the potassium salts of this acid.
.....................…..…......... .............……................... ...............….................... (1)
(e) 25.0 cm3 of a solution of phosphoric(V) acid of concentration 0.0590 mol dm–3 required
28.1 cm3 of 0.105 mol dm–3 sodium hydroxide for reaction in a titration, using
phenolphthalein indicator.
(i) Calculate the number of moles of phosphoric(V) acid in 25.0 cm3.
NT Exampro 27
(ii) Calculate the number of moles of sodium hydroxide in 28.1 cm3.
(iii) Write a balanced equation for the reaction of phosphoric(V) acid with sodium
hydroxide under these conditions.
.......................................................................................................................... (4)
(Total 15 marks)
23. (i) Define pH. .......................................................................................................…….
(ii) Define Kw, the ionic product of water.
........................................................................................................................……… (Total 2 marks)
24. (i) What is the principal property of a buffer solution?
...................................................................……….....................................................
...................................................................……….....................................................
(ii) The dissociation constant for ethanoic acid is 1.80 × 10–5 mol dm–3. Calculate the pH of
a buffer solution which has a concentration of 0.0150 mol dm–3 with respect to ethanoic
acid and 0.0550 mol dm–3 with respect to sodium ethanoate.
(Total 5 marks)
25. Hard water contains dissolved calcium or magnesium ions, which come from the rocks over
which the water has flowed. The concentration of these ions can be found in several ways.
(a) One method is to titrate a known volume of the water with a standard solution of the
compound edta.
Edta complexes with Ca2+ ions, as well as many transition metal ions, in a 1:1 ratio. To
ensure that the edta complexes satisfactorily with the Ca2+ ions, the solution must be
buffered at about pH 10. At this pH any indicator used will change colour when all the
Ca2+ ions have been complexed. Some other divalent ions, notably Fe2+, will also
complex with edta at this pH, but this can be eliminated if a solution of potassium cyanide
is added first.
(i) Explain the term ‘buffered at pH 10’.
NT Exampro 28
(ii) The buffer which is usually used in this titration is a mixture of aqueous ammonia
and ammonium chloride solution. Explain how this mixture behaves as a
buffer solution.
(iii) A given water sample contains Ca2+ and Fe2+ as the only divalent ions. Suggest
why the addition of cyanide ions, CN–, prevents the reaction of edta with Fe2+ ions.
(iv) A 50.0 cm3 sample of tap water was titrated with edta solution of concentration
0.0100 mol dm–3; 31.2 cm3 was needed before the indicator changed colour. What
is the Ca2+ ion concentration in the water in mol dm–3? (13)
(b) A second method for the determination of Ca2+ ion concentration uses the precipitation of
the salt calcium ethanedioate; this is filtered off, dissolved in warm dilute nitric acid, and
the ethanedioate now in solution as ethanedioic acid is determined by titration with
standard potassium manganate(VII) solution.
(i) 25.0 cm3 of a solution of Ca2+ ions containing 0.0500 mol dm–3 of Ca2+ ions was
treated with excess ammonium ethanedioate solution, and the precipitate of calcium
ethanedioate was filtered off. Find the mass of the salt which would be
precipitated.
(ii) The precipitate was washed with warm dilute nitric acid until completely dissolved,
and the washings made up to 250 cm3 with pure water. 25.0 cm3 portions of this
solution were added to about 25 cm3 of dilute sulphuric acid, and the mixture was
titrated at 60°C with 0.0200 mol dm–3 potassium manganate(VII) solution. Write
equations for the reactions occurring, and calculate the volume of potassium
manganate(VII) solution which would be required.
(iii) The first addition of potassium manganate(VII) loses its colour slowly.
Subsequent additions decolorise rapidly. Suggest a reason for this. (12)
(Total 25 marks)
26. Phosphoric(V) acid, H3PO4 , is a tribasic acid.
Write the formulae of the potassium salts of this acid.
.....................…....…..…...... .............…….........…........... ...............…...............……...
