CIE Jun 2017 v3 Paper 3 Q1 (with explained solutions) |

Skip to content

CIE Jun 2017 v3 Paper 3 Q1

Answers available below

Sodium hydrogencarbonate, NaHCO3, is used as baking soda in cooking. Baking soda may also contain small amounts of other chemicals. In this experiment, you will determine the percentage purity by mass of an impure sample of NaHCO3 by titration with sulfuric acid. PMT FA 1 is 0.0500 mol dm3 sulfuric acid, H2SO4. FA 2 is impure NaHCO3. methyl orange (a) Method Preparing a solution of FA 2Weigh the stoppered container of FA 2. Record the mass in the space below. Tip all the FA 2 into the beaker. Reweigh the container with its stopper. Record the mass. Calculate and record the mass of FA 2 used. Add approximately 100 cm3 of distilled water to the FA 2 in the beaker. Stir the mixture with a glass rod until all the FA 2 has dissolved. Transfer this solution into the 250 cm3 volumetric flask. Wash the beaker with distilled water and transfer the washings to the volumetric flask. Rinse the glass rod with distilled water and transfer the washings to the volumetric flask. Make up the solution in the volumetric flask to the mark using distilled water. Shake the flask thoroughly. This solution of impure NaHCO3 is FA 3. Label the flask FA 3. Results TitrationFill the burette with FA 1. Pipette 25.0 cm3 of FA 3 into a conical flask. Add several drops of methyl orange. Perform a rough titration and record your burette readings in the space below.UCLES 2017 9701/33/M/J/17 The rough titre iscm3.<br />
Carry out as many accurate titrations as you think necessary to obtain consistent results. Make sure any recorded results show the precision of your practical work. Record in a suitable form below all of your burette readings and the volume of FA 1 added in each accurate titration. PMT Keep FA 1 for use in Question 2. II III IV VI VII VIII [8] (b) From your accurate titration results, obtain a suitable value for the volume of FA 1 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 3 requiredcm3 of FA 1. [1] (c) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of sulfuric acid present in the volume of FA 1 calculated in (b). moles of H2SO4 =mol (ii) Balance the equation for the reaction of sulfuric acid and sodium hydrogencarbonate. State symbols are not required. NaHCO3 + H2SO4 Na2SO4 + CO2 + H2O (iii) Using your answers to (i) and (ii), calculate the number of moles of sodium hydrogencarbonate used in each titration.UCLES 2017 9701/33/M/J/17 [Turn over moles of NaHCO3 =mol<br />
(iv) Using your answer to (iii), calculate the mass of sodium hydrogencarbonate present in the mass of FA 2 used to prepare FA 3. PMT (v) Calculate the percentage purity by mass of the impure sodium hydrogencarbonate sample, FA 2. mass of NaHCO3 =g percentage purity by mass of impure NaHCO3, FA 2 =% (vi) What did you assume about the impurities in FA 2 when you calculated the percentage purity?(vii) A volumetric flask was labelled 250.00.10 cm3. Calculate the maximum percentage error when using this volumetric flask. maximum percentage error =% [7] [Total: 16]UCLES 2017 9701/33/M/J/17<br />

Show answer

9701/33 Question 1(a) Cambridge International AS/A LevelMark Scheme PUBLISHED Answer I Correct headings The following data are recorded in the space providedmass of container with FA 2mass of (empty) containermass of FA 2 Mass must be stated for each piece of data. Unit / g (etc.) must be given for each piece of data. Subtraction for mass of FA 2 used must be correct. II All the following data are recordedtwo burette readings and titre for the rough titration initial and final burette readings for two (or more) accurate titrations III Titre values recorded for accurate titrations, and Appropriate headings and units in the accurate titration tableinitial / start (burette) reading / volume final / end (burette) reading / volume titre or volume / FA 1 and used / added unit: / cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recorded IV All accurate burette readings are recorded to the nearest 0.05 cm3. The requirement to record to 0.05 applies to burette readings, including 0.00 cm3 (if this was the initial reading), but it does not apply to the titre. This mark is not awarded if:more than one final burette reading is 50.(00)50(.00) is used as an initial burette reading any burette reading is greater than 50.(00) V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre.Do not include a reading if it is labelled rough.Do not award the mark if any accurate burette readings (apart from initial 0 cm3) are given to zero dp.UCLES 2017 Page 2 of 10 May/June 2017 Marks<br />
9701/33 Question Cambridge International AS/A LevelMark Scheme PUBLISHED Answer May/June 2017 Marks two (or more) accurate identical titres (ignoring any that are labelled rough), then two (or more) accurate titres within 0.05 cm3, then two (or more) accurate titres within 0.10 cm3, etc. For assessment of accuracy (Q) marks, each Examiner should round any burette readings to the nearest 0.05 cm3, check subtractions and then select the best titres using the hierarchy:These best titres should be used to calculate the mean titre, expressed to nearest 0.01 cm3. Calculate the candidates ratio to 1 dp, as shown below. Ratio = correct mean titrecorrect mass Calculate the difference () between the candidates ratio and the supervisors ratio. Accuracy marks are awarded as follows. Award VI, VII and VIII if0.2 (cm3 g1) if 0.2 <0.4 if 0.4 <0.6Spread penalty: if the two best (corrected) titres used by the Examiner were0.50 cm3 apart, maximum 2 Award VI and VII accuracy marks. If only a rough titration is shown, award Q marks based on that, maximum 2 accuracy marks. 1(a) Award VI, only,UCLES 2017 Page 3 of 10<br />
9701/33 Question 1(b) Cambridge International AS/A LevelMark Scheme PUBLISHED Answer Candidate calculates the mean correctly.Candidate must take the average of two (or more) titres that are within a total spread of not more than 0.20 cm3.Working / explanation must be shown or ticks must be put next to the two (or more) accurate readings selected.The mean should be quoted to 2 dp, and be rounded to nearest 0.01 cm3. (e.g. 26.665 cm3 must be rounded to 26.67 cm3) Two special cases, where the mean need not be to 2 dp:Allow mean expressed to 3 dp only for 0.025 or 0.075 (e.g. 26.325 cm3)Allow mean if expressed to 1 dp, if all accurate burette readings (apart from initial 0) were given to 1 dp and the mean is exactly correct. (e.g. 26.0 and 26.2 = 26.1 is allowed) (e.g. 26.0 and 26.1 = 26.1 is wrongshould be 26.05) This mark is not awarded if:The rough titre was used to calculate the mean.The candidate did only one accurate titration.Burette readings were incorrectly subtracted to obtain any of the accurate titre values.All burette readings used to calculate the mean were recorded as integers Note: the candidates mean will sometimes be marked correct even if it was different from the mean calculated by the Examiner for the purpose of assessing accuracy. No of moles of H2SO4 used = 0.05(0)(b)/ 1000 to minimum 2 sf 2NaHCO3 + H2SO4 (cid:198) Na2SO4 + 2CO2 + 2H2O and No of moles of NaHCO3 = 2answer (i) 1(c)(i) 1(c)(ii) and 1(c)(iii)UCLES 2017 Page 4 of 10 May/June 2017 Marks<br />
9701/33 Question 1(c)(iv) Mass of NaHCO3 = answer (iii)1084 1(c)(v) % = answer (iv) / mass of FA 2 used100 Cambridge International AS/A LevelMark Scheme PUBLISHED Answer All answers attempted in (i), (iii), (iv) & (v) are shown to 3 or 4 sf Minimum 3 answers attempted to gain the mark 1(c)(vi) Any one of the following answers.the impurity does not react with (sulfuric) acid / FA 1 / NaHCO3 the impurity is not alkaline / acidic the impurity is neutral 1(c)(vii) % error (= 0.1 / 250100) = 0.04% May/June 2017 Marks 16 Total:UCLES 2017 Page 5 of 10<br />

Question 1 2 3

About us | Need Help? | Feedback

Proudly powered by WordPress | Theme: Sydney by aThemes.