OCR A AS Specimen Paper 2 Q6 (with explained solutions) |

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OCR A AS Specimen Paper 2 Q6

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6 A student carries out an experiment to identify an unknown carbonate. 15The student weighs a sample of the solid carbonate in a weighing bottle.The student tips the carbonate into a beaker and weighs the empty weighing bottle.The student prepares a 250.0 cm3 solution of the carbonate.The student carries out a titration using 25.0 cm3 of this solution measured using a pipette with 0.100 mol dm3 hydrochloric acid in the burette. (a) The sample of carbonate is dissolved in approximately 100 cm3 of distilled water in a beaker and the solution transferred to a volumetric flask. The volume of the solution is made up to 250.0 cm3 with distilled water. Another student suggests two possible sources of error:A small amount of solid remained in the weighing bottle.A small amount of solution remained in the beaker. State whether the other students statements are correct. How could the procedure be improved? Specimen[2]OCR 2014 H032/02 Turn over<br />
16 (b) The student carries out the final part of the experiment by adding 0.100 mol dm3 hydrochloric acid to a burette and performing a titration using a 25.0 cm3 sample of the aqueous carbonate. The student reads the burette to the nearest 0.05 cm3. The diagrams below show the initial burette reading and the final burette reading. initial reading final reading (i) Record the students readings and the titre. Specimen (ii) Describe what the student should do next to obtain reliable results for the titration. . [1] .[1]OCR 2014 H032/02<br />
17 (c) The equation below represents the reaction between the carbonate and hydrochloric acid. (i) Calculate the amount, in mol, of M2CO3 used in the titration. M2CO3(aq) + 2HCl(aq)2MCl(aq) + CO2(g) + H2O(l) n(M2CO3) =mol [2] 14.92 13.34 (ii) The students mass readings are recorded below. Use the students results to identify the carbonate, M2CO3. Show all your working. Mass of weighing bottle + carbonate / g Mass of weighing bottle / g Specimen [4]OCR 2014 H032/02 Turn over<br />

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H032/02 Question (a) (b) (i) (i) n(HCl) = (0.100)(answer to (c)(i)/1000) = 0.00222 (mol) ALLOW ECF from (b)(i) Mark Scheme June 20XX Marks Guidance Answer Not correct about the solid remaining in the weighing bottle (weighed by difference) AND Correct about the solution in the beaker (cid:57) Rinse out the beaker with distilled water and transfer to the volumetric flask before making up to 250 cm3 (cid:57) Initial reading = 0.60 (cm3) Final reading = 22.80 (cm3) Titre = 22.20 cm3 Initial and final values recorded to two decimal places AND titre recorded to the nearest 0.05 cm3 with correct units (cid:57) (ii) Suggests repeating the titration to obtain consistent/concordant results (those that agree to within 0.1 cm3) AND calculating the mean titre (cid:57) (c) (cid:57) n(M2CO3) = 0.00222/2 = 0.00111 (mol) (cid:57) Specimen 17 P M T<br />
(ii) n(M2CO3) in total = 0.00111 x 10 = 0.0111 mol (cid:57) Answer Molar mass = 1.58/0.0111 = 142.3 g mol1 (cid:57) Mass of M = (142.360)/2 = 41.15 (= K) (cid:57) K2CO3 (cid:57) H032/02 Question Mark Scheme June 20XX Marks Guidance Note: molar mass is between K2CO3 (138.2) and SrCO3 (147.6); only possible match for a Group 1 carbonate is K2CO3. 10 Total Specimen 18 P M T<br />

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