Answer all the questions in the spaces provided1 (a) Write an expression for Kw.

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(b) Use your expression to help you calculate the pH of 0.200 mol dm3 NaOH(aq).

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(c) The pH of 0.200 mol dm3 NH3(aq) is 11.3. Explain why this answer differs from youranswer in (b).

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(d) A 20.0 cm3 sample of 0.200 mol dm3 NH3(aq) was titrated with 0.100 mol dm3 HCl.On the following axes, sketch how the pH changes during this titration. Mark clearlywhere the end point occurs.

[3]

volume of added acid / cm3

pH

14

7

00 10 20 30 40 50 60

1

(e) From the following list of indicators, put a tick in the box by the side of the indicator youconsider most suitable for this titration.

[1]

(f) A solution containing NH3(aq) and NH4Cl(aq) acts as a buffer solution, resistingchanges in pH when acids or alkalis are added.

Explain with the help of equations how this mixture acts as a buffer.

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[Total: 10]

indicator pH at which colourchanges

place one tick onlyin this column

methyl violet

methyl orange

bromothymol blue

phenolphthalein

0.0 - 1.6

3.1 - 4.4

6.0 - 7.6

8.3 - 10.0

Group -1 Name_______________________________________Id.no._________A2-Level

2 2. Calcium hydroxide, Ca(OH)2, is slightly soluble in water.

(i) Write an expression for Ksp for calcium hydroxide, and state its units.

Ksp = units ............................................

(ii) 25.0 cm3 of a saturated solution of Ca(OH)2 required 21.0 cm3 of 0.0500 mol dm3 HCl for complete neutralisation.

Calculate the [OH(aq)] and the [Ca2+(aq)] in the saturated solution, and hence calculate a value for Ksp.

[OH(aq)] = .................................

[Ca2+(aq)] = ................................

Ksp = .........................................................................................

(iii) How would the solubility of Ca(OH)2 in 0.1 mol dm3 NaOH compare with that in water?

Explain your answer.

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[6]3 (a) Hydrogen fluoride, HF, behaves as a weak acid in water, with Ka = 5.6 104 mol dm

Calculate the pH of a 0.050 mol dm3 solution of HF.

pH = ....................................... [2]

(b) Gaseous ammonia and hydrogen fluoride react together to give solid ionic ammonium fluoride.

NH3(g) + HF(g) NH4F(s) H = 147 kJ mol1

(i) What type of reaction is this?

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3 (ii) Draw dot-and-cross diagrams (outer shells only) describing the bonding in the three compounds involved in this reaction.

NH3

HF

NH4F

(iii) There are three types of bonding in NH4F. Give the names of each of the three types, and state where in the compound each

type occurs.

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(c) (i) Write the expression for Kw.

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(ii) Use your expression and the value of Kw in the Data Booklet to calculate the pH of 0.150 mol dm3 NaOH(aq).

(iii) The pH of a 0.150 mol dm3 solution of piperidine is 11.9.

HN

piperidine

Suggest why this answer differs from your answer in (c)(ii).

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(iv) How would you expect the basicity of piperidine to compare to that of ammonia? Explain your reasoning.

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(d) 20.0 cm3 of 0.100 mol dm3 HCl was slowly added to a 10.0 cm3 sample of 0.150 mol dm3 piperidine. The pH was measured throughout the addition.

(i) Calculate the number of moles of HCl remaining at the end of the addition.

moles of HCl = .....................

(ii) Hence calculate the [H+] and the pH at the end of the addition.

pH = .....................

(iii) On the following axes, sketch how the pH will change during the addition of a total of 20.0 cm3 of 0.100 mol dm3 HCl. Mark clearly where the end point occurs.

0 5 10

volume HCl added / cm315 20

14

7

0

pH

(iv) From the following list of indicators, put a tick in the box by the side of the indicator most suitable for this titration.

indicator pH at whichcolour changesplace one tick only

in this column

A 0 - 1

B 3 - 4

C 11 - 12

D 13 - 14

[6]

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4