AS Edexcel Chemistry Formulae and Equations

8/10/2019 AS Edexcel Chemistry Formulae and Equations

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Q3.Sodium burns in oxygen to give a pale yellow solid X.(a) (i) 1.73 g of sodium reacts with 1.20 g of oxygen.

Calculate the empirical formula of X.(2)

(ii) The molar mass of Xis 78 g mol1. Give the molecular formula of X.(1)

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(iii) Write the equation, including state symbols, for the reaction of sodium with oxygen toproduce X.

(2)

(iv) Calculate the volume of oxygen in dm3(at room temperature and pressure) whichreacts with1.73 g of sodium. (The molar volume of any gas at room temperature andpressure is 24 dm3mol1.)

(2)

(v) Calculate the number of oxygen moleculesthat react with 1.73 g of sodium.(The Avogadro constant = 6.02 1023mol1.)

(1)

>(b) If sodium is burnt in air, compound Xis not the only product. Suggest why this is so.(1)

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(Total for question = 9 marks)


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Q4.Sulfamic acid is a white solid used by plumbers as a limescale remover.

(a) Sulfamic acid contains 14.42% by mass of nitrogen, 3.09% hydrogen and 33.06% sulfur. Theremainder is oxygen.

(i) Calculate the empirical formula of sulfamic acid.

(3)

(ii) The molar mass of sulfamic acid is 97.1 g mol1. Use this information to deduce

the molecular formula of sulfamic acid.

(1)

(b) A solution of sulfamic acid contains hydrogen ions. The hydrogen ions react withmagnesium to produce hydrogen gas. In an experiment, a solution containing5.5 x 103moles of sulfamic acid was reacted with excess magnesium. The volumeof hydrogen produced was 66 cm3, measured at room temperature and pressure.

(i) Draw a labelled diagram of the apparatus you would use to carry out thisexperiment, showing how you would collect the hydrogen produced andmeasure its volume.

(2)

(ii) Calculate the number of moles of hydrogen, H2, produced in this reaction.

[The molar volume of a gas is 24 dm3mol1at room temperature and pressure]

(1)

(iii) Show that the data confirms that each mole of sulfamic acid produces one mole


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(d) Show that the calculated amounts of magnesium, hydrochloric acid and hydrogen areconsistent with the following equation for the reaction

(1)(e) Calculate the maximum mass of magnesium chloride that would be formed in this

reaction. Give your answer to threesignificant figures.(3)

(Total for Question = 7 mark)


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Q2.(a) (i)The majority of candidates were able to calculate the moles of carbon dioxide. If the volumeof the gas used was left in units of cm

3(as in the question), then the molar volume had to

be converted from 24 dm3mol1into 24 000 cm3mol1. If the answer given was incorrect,the reason was that the candidates had not worked with consistent units.

Results Plus: Examiner CommentsThis is a correct answer, clearly set out.

Results Plus: Examiner TipIf the volume of gas is in cm3, then the molar volume must be converted from 24 dm3mol1to 24 000 cm3mol1.

(a) (ii-ii i)Many candidates were able to calculate the mass of sodium hydrogencarbonate presentin the sample. The most frequent error in (a)(ii) was to calculate the moles of impuresodium hydrogencarbonate instead of the mass required by the question. Under these

circumstances, however, the two marks in (a)(iii) could be retrieved by using the moles in(a)(i) in the subsequent calculation. A mark was often lost in (a)(iii) by not giving the finalanswer to two significant figures.


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Results Plus: Examiner CommentsWorking has been shown and the answer in (a)(iii) has been given to the requireddegree of accuracy.

Results Plus: Examiner TipCheck your final answer carefully when a specific degree of accuracy is required by thequestion.

(b )Part (b)(i) was generally well-answered. The most frequent error was to confuse a gassyringe with a burette and to double the total error given in the question.


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For (b)(ii), the majority of candidates realised that carbon dioxide is slightly soluble in water.A significant number of answers, however, mistakenly thought that because water was aproduct of the acid-hydrogencarbonate reaction, this in itself would affect the accuracy ofthe experiment.

Results Plus: Examiner CommentsBoth responses (b)(i) and (b)(ii) are correct.

Results Plus: Examiner TipNote that the total error has been given in the question for (b)(i).


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Q3.(a)(i)

Many candidates calculated the empirical formula correctly as NaO.

Results Plus: Examiner Comments1:1 ratio of Na:O had not been made explicit, but the correct answer NaO was given so

both marks were awarded.

Results Plus: Examiner TipAlways show every step in your working.

(a)(ii)

A surprising number of candidates gave a molecular formula for a compound for which the molarmass was not 78 g mol1.

(a)(iii)

Many candidates were awarded both marks, but a significant number gave the state symbol forNa2O2as (aq) rather than (s).

Results Plus: Examiner CommentsThis is a correctly balanced equation, with the state symbols which are also correct, theanswer scored both marks.

Results Plus: Examiner TipAlways consider state symbols carefully when asked for them in an equation.

(a)(iv)

Most candidates gained the second scoring point which required the calculated moles of oxygen,O2, to be multiplied by 24 dm

3mol1in order to obtain the volume of gas in units of dm3. Therewas some confusion, however, between calculating moles of oxygen atoms, O(g), and moles ofoxygen molecules, O2(g).


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Results Plus: Examiner CommentsThis is a well laid-out answer which scored both marks.

Results Plus: Examiner TipSet out your answers to calculation questions in an orderly way as illustrated in theabove response.

(a)(v)

This question was generally well answered, although sometimes candidates chose to calculatethe number of oxygen atoms instead of molecules.

Results Plus: Examiner CommentsThis was the correct response, which scored the mark available.

Results Plus: Examiner TipAlways check which type of particle (atom, ion or molecule) is being referred to in thequestion.

(b )

This proved difficult for the majority of candidates. A significant number of responses includedincorrect references to the presence of hydrogen gas, H2, in air.


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The diagrams in (b)(i) had to show a workable method of collecting hydrogen, and a suitablemethod ofmeasuring its volume. The diagram had to be good enough to show another person what to do,but tubing,bungs etc were not expected to be drawn to text-book standard. A number of diagrams showedhydrogen

being collected in test tubes with no way of measuring the volume. More major errors suggesteda lack ofpractical experience. Collection over water often showed worrying disregard for water levels orindeed theneed for water at all! There were diagrams showing a gas being produced in a sealed container(other thana syringe) connected to a sealed flask, or with gas being produced in an open container fromwhich it wouldhave escaped, with a side arm leading to a collecting vessel.

Results Plus: Examiner TipDiagrams should be clear enough for someone to work out how to set up apparatus,and they should show workable methods.

Results Plus: Examiner Comments

This gets one mark for showing a workable method of collecting hydrogen, but the testtube should be replaced by a measuring cylinder or inverted burette. The clamp standsneed not be shown in diagrams like this.


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Results Plus: Examiner Comments

This scores both marks. The syringe is not very well drawn but it is labelled.

The number of moles of hydrogen in (b)(ii) was usually correct, the main error being to forget toconvertthe molar volume to cm3. In (b)(iii) even those candidates who showed that two moles of sulfamicacidproduced one mole of hydrogen found it difficult to see that each mole of hydrogen wasequivalent to twohydrogen ions.

Candidates found the ionic equation in (c)(i) challenging. Candidates need to learn to balance

both atomsand charges when writing these. Three possible answers were allowed. The equation couldshow eitherformation of water and carbon dioxide, formation of carbonic acid, or formation of hydrogencarbonate ions.

Results Plus: Examiner TipBoth atoms and charges must be balanced in the equation.

Results Plus: Examiner CommentsThis doesn't get the mark because the charge on the carbonate ion should be 2-. If theproducts are known, it is possible to work out that another negative charge is neededfor balance.

In (c)(ii) few candidates thought about the advantages of a descaling agent being a solid, andmost focussedon the acid strength or corrosive properties of the two acids.


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Q5.(a)Most candidates found the calculations in this question straightforward and a higherproportion were able to round their final answer in (e) correctly than has been the casewith similar questions in earlier papers in this series. However, a significant number of

candidates still expressed their answers to the first three parts of the question incorrectlywith excessive and incorrect rounding being the common errors. While the stoichiometricratio question seemed well understood by most candidates, the mark was often lost byeither failing to make the necessary link between the calculation and the equation or lookingat only two figures, usually the magnesium and the acid.


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Results Plus: Examiner CommentsAn excellent answer although the candidate has chosen to use the Arof magnesiumgiven in the Periodic Table rather than the approximate value stated in the question.


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Results Plus: Examiner CommentsThis candidate clearly understands how to do this question but in (d) fails to completethe answer, only considering the stoichiometric ratio of magnesium to hydrogen. Notealso the use of a rounded value of the number of moles in (e).

Results Plus: Examiner TipIt is good practice to retain in your calculator unrounded values obtained in intermediatesteps of a calculation and to use these rather than the rounded values that you writedown.


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1.3i Markscheme

Q1 D

Q2


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Q4.


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Q5.

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AS Edexcel Chemistry Formulae and Equations