VJC Prelim P4 Answers

1  VJC 2017 9729/04/PRELIM/17 [Turn over Victoria Junior College 2017 H2 Chemistry Prelim Exam 9729/4 Suggested Answers Answer all the questions in the spaces provided. 1 Determination of the Mr of a carbonate salt FA 1 is a solid carbonate, XCO3. FA 2 is a solution containing 1.00 mol dm-3 of hydrochloric acid, HCl. FA 3 is a solution of NaOH of concentration 0.100 mol dm–3. In this question, you will perform a titration. The data from this titration will be used to determine  the amount of CO32– ions in the mass of FA 1 used,  the Mr of XCO3 and hence the identity of the metal X. (a) Titration of FA 4 against FA 3 (i) You are to determine the amount of carbonate ions, CO 32–, in FA 1, by back titration after some FA 2 has been added to it. Carry out the procedure as listed below. 1. Weigh accurately between 1.25 – 1.35 g of FA 1 into a clean and dry 150 cm 3 beaker. If you have used the TARE facility on the balance, indicate clearly in your recording. 2. Record your we ighing in an appropriate format in the space provided on the next page. 3. Measure out 50 cm 3 of FA 2 into a measuring cylinder. Pour t his into the beaker containing FA 1. Stir thoroughly to ensure all of FA 1 has dissolved. 4. Quantitatively transfer the mixt ure into a 250 cm 3 volumetric flask. Make up to the mark with distilled water. Shake to obtain a homogeneous solution. Label this solution FA 4. 5. Pipette 25.0 cm 3 of FA 4 into a conical flask and add 2 drops of methyl orange indicator. 6. Fill the burette with FA 3. 7. Titrate FA 4 with FA 3. 8. Repeat the titration as many times as you consider necessary to obtain accurate results. 9. Record your titration results in the space provided on the next page . Make certain that your recorded results show the precision of your working.

2  VJC 2017 9729/04/PRELIM/17 [Turn over Results: For weighing of FA 1 Mass of beaker and FA 1 / g 5.125 Mass of empty beaker / g 3.825 Mass of FA 1 used / g 1.300 OR if TARE is used: Mass of beaker only / g TARE Mass of FA1 / g 1.300 Table with appropriate headers / units AND all readings to 3 dp AND mass used within 1.25 – 1.35 g Titration Experiment 1 2 Final burette reading / cm3 25.90 36.00 Initial burette reading / cm3 0.00 10.00 Volume of FA 3 used / cm3 25.90 26.00 Table with appropriate headers and units Record all burette readings to 2 dp At least two consistent readings ±0.10 cm3 Accuracy: [5m] [2m] Mr within ± 4 units of supervisor’s answer (108) [1m] Mr within ± 8 units of supervisor’s answer (108) [6] (ii) From your titrations, obtain a suitable volume of FA 3 to be used in your calculations. Show clearly how you obtained this volume. Average volume of FA 3 = ( 25.90 + 26.00) / 2 = 25.95 cm3 volume of FA 3 = 25.95 cm3 [1] (b) (i) Calculate the amount of excess acid pipetted into the conical flask. Amount of acid pipetted in 25.0 cm3 = (25.95 / 1000) x 0.100 = 2.60 x 10–3 mol Amount of excess acid in conical flask =2.60 x 10–3 mol [1] (ii) Calculate the total amount of excess acid in the volumetric flask. Total amount of excess acid = (250 / 25.0) x 2.60 x 10–3 mol = 2.60 x 10–2 mol amount of excess acid in the volumetric flask = 2.60 x 10–2 mol [1]

3  VJC 2017 9729/04/PRELIM/17 [Turn over (iii) Hence calculate the amount of acid that has reacted with the carbonate that you have weighed out. Amount of acid reacted = (1.00 x 50/1000) – 2.60 x 10–2 = 0.0240 mol Amount of acid reacted = 0.0240 mol [1] (iv) Use your answer in b(iii) to calculate the Mr of XCO3. XCO3 + 2HCl  XCl2 + CO2 + H2O Amount of XCO3 = (0.0240 / 2) = 0.0120 mol Mr of XCO3 = (1.30 / 0.0120) = 108.3 Mr of XCO3 = 108.3 Deduce the identity of metal X. Show your working. [Ar: C, 12.0; O, 16.0; H, 1.0; Be, 9.0; Mg, 24.3; Ca, 40.1; Sr, 87.6; Ba, 137.3] Ar of X + 12.0 + 3(16.0) = 108.3 Ar of X = 48.3 Hence X is Ca (calculated value closest to Ar of Ca) X is Ca [3] [Total: 13] (c) Planning Sodium carbonate, Na 2CO3, does not decompose on heating with a Bunsen burner. Sodium hydrogencarbonate, NaHCO3, decompose on heating. 2NaHCO3(s)  Na2CO3(s) + H2O(g) + CO2(g) You are to design an experiment in which the percentage by mass of NaHCO3 in a mixture of NaHCO 3 and Na 2CO3 can be determined by heating and weighing alone. The only apparatus available consists of: a boiling tube and holder a chemical balance a Bunsen burner a heat proof mat You are to show how you would use the results of this experiment to determine the percentage by mass of NaHCO3 in the mixture.

4  VJC 2017 9729/04/PRELIM/17 [Turn over (i) Outline, step by step, the practical sequence for the method you would use to:  make appropriate weighings,  decompose the sodium hydrogencarbonate in the mixture by heating,  ensure that the decomposition was complete. 1. Weigh the clean and dry boiling tube. 2. Add all the sample into the boiling tube. Weigh the mass of the boiling tube with the sample. 3. Heat the boiling tube gently at first (~1 min) and then strongly (~5 min) to decompose the sodium hydrogencarbonate. Heat the whole length of the tube to ensure even heating of the mixture and to prevent condensation. 4. Cool the boiling tube on a heat proof mat, and weigh the mass of the boiling tube and its residue. 5. Repeat heating, cooling and weighing steps [step s 3 and 4 ] until constant mass is achieved (OR consecutive mass difference within ±0.050 g is achieved) to ensure complete decomposition. Record all masses. Mark Scheme [1m]: weigh empty boiling tube, sample and boiling tube before and after heating [1m]: give details such as dry boiling tube, heat strongly, heat whole length of tube, use of heat proof mat for cooling [1m]: heat until constant mass (OR within ±0.050 g) to ensure complete decomposition [3] (ii) By considering the products of the decomposition, suggest a reason why a crucible, without a lid, might be more appropriate than a boiling tube for this experiment. The crucible has a wider surface area for the volatile substances such as water vapour and carbon dioxide to escape more readily which lowers the risk of breakage due to condensation. [OR It can withstand high temperature during heating which reduces the risk of breakage.] [1]

5  VJC 2017 9729/04/PRELIM/17 [Turn over (iii) Prepare a table to show the masses you would measure and record during the experiment. Include in your table any other masses y ou would calculate from the experimental results to enable you to determine the percentage by mass of NaHCO3 in the mixture. Insert in your table the letters A, B, C etc. to represent each mass. Use these letters to show how your calculated masses are ob tained e.g. B – A. Mass of empty boiling tube / g A Mass of boiling tube and sample before heating / g B Mass of boiling tube and residue after heating / g C Mass of sample used / g B – A Mass loss due to H2O and CO2 / g B – C Mark Scheme [1m]: Tabulation with headers and units as well as raw data recorded (first 3 rows) [1m]: Processed data of mass of sample used and mass loss due to heating (last 2 rows) [2] (iv) Use the letters you have entered in (c)(iii) to show how you would process the results to find:  the mass of NaHCO3 in the mixture, [Ar: C, 12.0; H, 1.0; O, 16.0; Na, 23.0] Mass of NaHCO3 in the mixtu