VJC Prelim P4 Questions

2  VJC 2017 9729/04/PRELIM/17 [Turn over 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 weighing 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 this into the beaker containing FA 1. Stir thoroughly to ensure all of FA 1 has dissolved. 4. Quantitatively transfer the mixture 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.

3  VJC 2017 9729/04/PRELIM/17 [Turn over Results: For weighing of FA 1 Titration [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. volume of FA 3 = ………………….. [1] (b) (i) Calculate the amount of excess acid pipetted into the conical flask. amount of excess acid in the conical flask = …………………… [1] (ii) Calculate the total amount of excess acid in the volumetric flask. amount of excess acid in the volumetric flask = …………………… [1]

4  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] (iv) Use your answer in b(iii) to calculate the Mr of XCO3. Mr of XCO3 = …………………….. 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] X is ………………………………… [3]

5  VJC 2017 9729/04/PRELIM/17 [Turn over (c) Planning Sodium carbonate, Na 2CO3, does not decompose on heating with a Bunsen burner. Sodium hydrogencarbonate, NaHCO3, decomposes 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. (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. …………………………………………………………………………………… …………………………………………………………………………………… …………………………………………………………………………………… …………………………………………………………………………………… …………………………………………………………………………………… …………………………………………………………………………….……… …………………………………………………………………………….……… ……………………………………………………………………………….…… ……………………………………………………………………………….…… ……………………………………………………………………………….…… ……………………………………………………………………………….…… ……………………………………………………………………………….…… ……………………………………………………………………………….…… ………………………………………………………………………………….… ………………………………………………………………………………… [3]

6  VJC 2017 9729/04/PRELIM/17 [Turn over (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. ………………………………………………………………………………….… ………………………………………………………………………………… [1] (iii) Prepare a table to show the masses you would measure and record during the experiment. Include in your table any other masses you 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 l etters A, B, C etc. to represent each mass. Use these letters to show how your calculated masses are obtained e.g. B – A. [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]  the percentage by mass of NaHCO3 in the mixture. [2] [Total: 21]

7  VJC 2017 9729/04/PRELIM/17 [Turn over 2 Determination of the concentration of a mixture of acids and the enthalpy change of neutralization FA 5 is an aqueous solution prepared by mixing equal volumes of y mol dm –3 hydrochloric acid, HCl, and y mol dm–3 sulfuric acid, H2SO4. FA 6 is 1.60 mol dm–3 sodium hydroxide, NaOH. Take care as aqueous solutions of sodium hydroxide are corrosive. When an acid is run into an alkali, an exothermic reaction takes place and the temperature of the mixture increases until the end –point is reached. If acid is added beyond the end –point, the temperature will decrease as no further reaction takes place and the acid is at a lower temperature than the mixture. You are to follow the neutralisation of the acids in FA 5 by measuring the temperature as volumes of FA 5 are added in regular portions from a burette to a fixed volume of FA 6 placed in a Styrofoam cup. The data obtained will enable you to determine the value of y and the enthalpy change of neutralization, ∆Hneut, of this acid–base reaction. In an appropriate format in the space provided on the next page, prepare a table in which you may record the following after each addition of FA 5:  Total volume of FA 5 added from the burette up to the point in time  Total volume of solution in the cup (Vcup)  Temperature measured (T) You also need to calculate the corresponding values of:  ΔT = T – T0, where T0 is the initial temperature of FA 6  (Vcup × ΔT) to 3 significant figures 1. Fill the burette to 0.00 cm3 with FA 5. 2. Place the Styrofoam cup in a 250 cm3 beaker to provide support for the cup. Use the measuring cylinder to place 40.0 cm 3 of FA 6 into the cup and measure the steady temperature of the alkali, T0. 3. Run 5.00 cm 3 of FA 5 from the burette into the cup, stir the solution with the thermometer and record the maximum temperature, T. 4. Immediately run a further 5.00 cm3 of FA 5 from the burette into the cup, stir and record the maximum temperature as before. 5. Continue the addition of FA 5 in 5.00 cm 3 portions and record the maximum or minimum t emperature reached after each addition. Do this until a total of 45.00 cm3 of solution have been run from the burette.

8  VJC 2017 9729/04/PRELIM/17 [Turn over (a) (i) Experimental results