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