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CIE Nov 2013 v4 Paper 3 Q2
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![Examiners For Use The percentage by mass of sodium carbonate in a mixture with sodium chloride can be estimated by adding a weighed sample of the mixture to a weighed excess of hydrochloric acid and measuring the mass of carbon dioxide evolved. Sodium chloride does not react with hydrochloric acid. FB 3 is 2.00 mol dm3 hydrochloric acid, HCl. FB 4 is a mixture of solid sodium carbonate, Na2CO3, and solid sodium chloride, NaCl. You are to determine the mass of carbon dioxide given off when the sodium carbonate in the mixture, FB 4, reacts with excess hydrochloric acid. (a) Method Record all weighings, in an appropriate form, in the space below.Use the measuring cylinder to transfer 75 cm3 of FB 3 into a 250 cm3 conical flask. Weigh the flask and acid, and record the mass. Weigh the labelled tube containing FB 4 and record the mass. Tip the FB 4 into the acid in the flask, a little at a time. When the reaction slows down, swirl the flask for 2 to 3 minutes. Reweigh the flask and its contents, and record the mass. Reweigh the tube labelled FB 4 with its stopper and any residual mixture not added to the acid, and record the mass. Calculate the mass of the mixture, FB 4, added to the acid. Record the mass of carbon dioxide given off in the reaction. This may be calculated using the following formula. mass of CO2 = (mass ofask + acid) + (mass FB 4 added)( nal mass ofask + contents) II III IV VI [6]UCLES 2013 9701/34/O/N/13 [Turn over<br />
Examiners For Use (b) The reaction of sodium carbonate with hydrochloric acid is shown in the equation. Na2CO3(s) + 2HCl (aq)2NaCl (aq) + CO2(g) + H2O(l) (i) Calculate the mass of sodium carbonate that reacts with the hydrochloric acid to give the mass of carbon dioxide recorded in (a). [Ar : C, 12.0; O, 16.0; Na, 23.0] (ii) Calculate the percentage by mass of sodium carbonate in FB 4. mass of Na2CO3 =g FB 4 contains% by mass Na2CO3 [2] (c) Mixtures of solids containing sodium carbonate can be analysed either by the procedure you used in Question 1 or the procedure you used in Question 2. The procedure used in Question 2 is likely to give a less accurate value for the percentage of sodium carbonate. (i) Suggest a signi cant source of error in the experimental method used in Question 2.(ii) State whether the error identi ed above would increase or decrease the calculated percentage by mass of Na2CO3 in the mixture. Explain your answer.(iii) Suggest an improvement to the experimental method or apparatus used in Question 2 that would reduce the error given in (i).[3] [Total: 11]UCLES 2013 9701/34/O/N/13<br />](https://learnah.org/wp-content/uploads/2018/08/November_2013_v4_QP_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
![Page 5 Mark Scheme Syllabus Paper GCE A LEVELOctober/November 2013 9701 34 Question Sections Indicative material Mark Total 2 (a) PDO I Presents data in list/form in the space provided. Layout Must have all four required weighings and attempt at mass of solid/FB 4 or attempt at mass of carbon dioxide. PDO II Gives all appropriate headings and units Recordingmass/weight of flask + acidmass of tube + FB 4mass of flask + contents (owtte)mass of tube + residue/mass of tubemass of FB 4mass of CO2/mass lost (minimum of four required pieces of information) Units: /g or (g) or in g or g by each entry (Ignore irrelevant data) II All recorded balance readings consistent to at least 1 decimal place. (minimum of three balance readings) IV Correctly calculates the mass of FB 4 added and the mass of carbon dioxide evolved. ACE Interpretation MMO Quality V and VI Calculate 4 FB mass of mass carbon dioxide to 3 significant figures and compare with Supervisor. Award V and VI for a difference0.20 Award V for a difference of 0.20 <0.50 [6]Cambridge International Examinations 2013<br />
Page 6 Mark Scheme Syllabus Paper GCE A LEVELOctober/November 2013 9701 34 Question Sections Indicative material Mark Total [2] 2 (b) ACE (i) Correctly calculates Interpretation from CO mass (a) 44 to 24 sf 106 (ii) Correct expression mass 4FB from (a) (b)(i) 100 or correct answer to 24 sf (Do not penalise sf twice.) 2 (c) (i) ACE Suggests a suitable significant source of error: InterpretationCO2 remains dissolved in acid Impossible to prevent all acid sprayReaction does not go to completion/ CO2 not diffused (owtte) from flaskSome FB 4 sticks to the wall of the flask. (ii) ACE Would lower %/decrease since less mass lost/CO2 lost/ Conclusions given out/evolved. Would raise %/increase since more mass lost. Would lower % since less mass lost/CO2 produced. Would lower % since less mass lost/used or less CO2 produced Explanation must follow source of error. If using 1dp balance then allow cannot tell whether the answer should be greater or smaller/% error could be either wayCambridge International Examinations 2013<br />
Page 7 Mark Scheme Syllabus Paper GCE A LEVELOctober/November 2013 9701 34 Question Sections Indicative material Mark Total 2 (c)(iii) ACE Improvement suggested must be linked to the error Improvement (even if imprecisely expressed) identified in (i). Dissolved CO2 Saturate the solution with CO2 before experiment or use warm acid or use less acid. Acid spray Use taller container or cotton wool plug or bung with hole or collect gas in syringe or use less concentrated acid. Going to completion Keep weighing until mass does not go down further or leave for longer or swirl for longer or warm flask and contents or use more concentrated acid. Sticks to side Use beaker or wider-necked flask. If 1dp balance used then allow use balance to 2 or 3 dp. [3] [Total 11]Cambridge International Examinations 2013<br />](https://learnah.org/wp-content/uploads/2018/08/November_2013_v4_MS_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
