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CIE Nov 2016 v4 Paper 3 Q2
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![In Question 1 you calculated the relative atomic mass, Ar, of magnesium by measuring the volume of hydrogen produced. The relative atomic mass can also be determined by investigating how much of the hydrochloric acid reacted with the magnesium. PMT The experiment described in Question 1 was repeated, this time using 0.21 g of magnesium ribbon and 30.0 cm3 of 1.00 mol dm3 hydrochloric acid. All the solution left in the burette and tub was kept and water added to make the total volume 250 cm3. This solution was labelled FB 3. You will titrate FB 3 using a known concentration of aqueous sodium carbonate to determine how much hydrochloric acid was left over after the reaction with magnesium. Na2CO3(aq) + 2HCl (aq)2NaCl (aq) + H2O(l) + CO2(g) FB 3 is the solution of hydrochloric acid described above. FB 4 is aqueous sodium carbonate containing 2.64 g dm3 Na2CO3. bromophenol blue indicator (a) MethodFill the burette with FB 3. Pipette 25.0 cm3 of FB 4 into a conical flask. Add about 10 drops of bromophenol blue indicator. Perform a rough titration and record your burette readings in the space below. The rough titre iscm3 Carry out as many accurate titrations as you consider necessary to obtain consistent results. Record, in a suitable form below, all of your burette readings and the volume of FB 3 added in each accurate titration. Make certain any recorded results show the precision of your practical work. II III IV VI VII [7]UCLES 2016 9701/34/O/N/16<br />
(b) From your accurate titration results, obtain a suitable value for the volume of FB 3 to be used in your calculations. Show clearly how you obtained this value. PMT 25.0 cm3 of FB 4 requiredcm3 of FB 3. [1] (c) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Use the information on page 4 and the Periodic Table on page 12 to calculate the number of moles of sodium carbonate in the 25.0 cm3 of FB 4 used in each titration. moles of Na2CO3 =mol (ii) Use your answer to (i) to calculate the number of moles of hydrochloric acid present in the volume of FB 3 recorded in (b). Na2CO3(aq) + 2HCl (aq)2NaCl (aq) + H2O(l) + CO2(g) (iii) Use your answer to (ii) to calculate the number of moles of hydrochloric acid present in 250 cm3 of FB 3. moles of HCl present =molUCLES 2016 9701/34/O/N/16 [Turn over moles of HCl present in 250 cm3 =mol<br />
(iv) Use the information on page 4 to calculate the number of moles of hydrochloric acid added to the magnesium. PMT (v) Calculate the number of moles of hydrochloric acid that reacted with the magnesium. moles of HCl added =mol moles of HCl that reacted with the magnesium =mol (vi) Use your answer to (v) and the mass of magnesium used to calculate the relative atomic mass, Ar, of magnesium. Mg(s) + 2HCl (aq)MgCl 2(aq) + H2(g) Ar of Mg =[5] (d) A solution of sodium hydroxide was prepared at the same concentration, in mol dm3, as FB 4. A student repeated the titration but replaced FB 4 with this solution of sodium hydroxide. (i) Explain the effect that replacing FB 4 with this solution of sodium hydroxide would have on the volume of acid, FB 3, needed for the titration.(ii) If the sodium hydroxide had been stored for a long time it would not be suitable for use to find the concentration of the acid. Suggest why storage for a long time would make the sodium hydroxide unsuitable.[2] [Total: 15]UCLES 2016 9701/34/O/N/16<br />](https://learnah.org/wp-content/uploads/2018/08/November_2016_v4_QP_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
![Page 4 Cambridge International AS/A LevelOctober/November 2016 Mark Scheme Syllabus 9701 Paper 34 PMT Question 1(d) Answer Volume (gas) measured/or moles/amount gas/H2 would have been less Ar greater but must follow from smaller moles of H2/Mg or Use correct molar volume for new room temperature Ar unchanged (but must follow from Vm smaller) Question Answer Total: 2(a) I Initial and final burette readings and volume added recorded for rough titre and initial and final reading for two (or more) accurate titrations II Initial and final burette readings and volume of FB 3 added recorded for each accurate titration. Headings and units correct for accurate titrations. Heading: initial/final (burette) reading/volume or reading/volume at start/finish and volume/FB 3 added/used or titre (not difference/amount/total unless total as an extra with volume used/ titre,) and Units: (cm3) or/ cm3 or in cm3 or cm3 by every entry Marks 1 1 or 1 1 10 MarksUCLES 2016<br />
Page 5 Question Cambridge International AS/A LevelOctober/November 2016 Mark Scheme Syllabus 9701 Paper 34 Answer Marks PMT 2(b) III All accurate burette readings recorded to the nearest 0.05 cm3 Do not award this mark if: 50(.00) is used as an initial burette reading; more than one final burette reading is 50(.00) any burette reading is > 50(.00) IV Final uncorrected titre is within 0.10 cm3 of any previous uncorrected accurate titre. V, VI and VII Examiner rounds any accurate burette readings to the nearest 0.05 cm3, checks subtractions and then selects the best accurate titres using the hierarchy: identical titres; titres within 0.05 cm3; titres within 0.1 cm3; etc., to calculate mean correct to 0.01 cm3. Examiner compares candidates titre value with that of the Supervisor. Award V, VI and VII for0.20 cm3 Award V and VI for 0.20 cm3 <0.30 cm3 Award V for 0.30 cm3 <0.50 cm3Candidate must average two (or more) titres where the total spread is0.20 cm3. Calculation of mean Check mean titre is correctly calculated from clearly selected values (ticks or working)Working must be shown or ticks must be put next to the two (or more) accurateThe mean should normally be quoted to 2 dp rounded to the nearest 0.01. [e.g. readings selected. 26.667 must be rounded to 26.67] Two special cases where the mean may not be to 2 dp: allow mean to 3 dp only for 0.025 or 0.075 e.g. 26.325; allow mean to 1 dp if all accurate burette readings were given to 1 dp (ignoring initial given 1 1 1UCLES 2016<br />
Page 6 Question Cambridge International AS/A LevelOctober/November 2016 Mark Scheme Syllabus 9701 Paper 34 Answer Marks PMT as 0) and the mean is exactly correct. [e.g. 26.0 and 26.2 = 26.1 is correct but 26.0 and 26.1 = 26.1 is incorrect.] Do not award this mark if:the rough titre was used to calculate the mean; the candidate carried out only 1 accurate titration; burette readings were incorrectly subtracted to obtain any of the accurate titre values; all burette readings (resulting in titre values used in the calculation of the mean) are integers. Note: the candidates mean will sometimes be marked as correct even if it is different from the mean calculated by the examiner for the purpose of assessing accuracy. 2(c)(i) Correctly calculates 2(c)(ii) and 2(c)(iii) 6.23 / 6.225 / 6.226 10 42.64106 40 Correctly uses (i)2 and (ii)(b) 250 2(c)(iv) Correctly calculates Moles HC = 30 1.001000 = 0.03(00)UCLES 2016<br />
Page 7 Question Cambridge International AS/A LevelOctober/November 2016 Mark Scheme Syllabus 9701 Paper 34 Answer Marks PMT 15 Total 2(c)(v) and 2(c)(vi) Correctly uses (iv)(iii) and 0.21 2 (v) Answers to 3 or 4 sf (minimum 4 answers attempted, allow 2 sf in (vi)) 2(d)(i) 2(d)(ii) Half the volume needed since 1:1 ratio/1 mole NaOH in equation (Impure) since absorbed/reacted with CO2 or water vapour/water from the airUCLES 2016<br />](https://learnah.org/wp-content/uploads/2018/08/November_2016_v4_MS_-_Paper_3_CIE_Chemistry_A-level_2-0.jpg)
