2019 ACJC Prelim H2 Chem P4 QP

3 © ACJC 2019 9729/04/Prelim/2019 [Turn over 1 Determination of solubility product, Ksp, of magnesium carbonate Magnesium carbonate is basic and sparingly soluble in water, resulting in its use as drying agents in sports and antacids. You are to carry out a titrimetric analysis to determine the solubility product of magnesium carbonate. A saturated solution of magnesium carbonate can be prepared by dissolving excess solid magnesium carbonate into a measured volume of water. The mixture will be left to stand for some time before filtering. The filtrate can then be analysed through titration with a standard solution of hydrochloric acid. The saturated solution of magnesium carbonate , FA 1, has been prepared for you, according to the procedure in (a). (a) Preparation of FA 1 1. Use a measuring cylinder to transfer 50 cm3 of deionised water into a 100 cm3 beaker. 2. Use a spatula to add solid magnesium carbonate into the same beaker, a little at a time with stirring until no more can dissolve. 3. Leave to stand for five minutes. 4. Filter the mixture to obtain the filtrate in a 250 cm 3 conical flask. Ensure that the filtrate is clear. Do not wash the residue. 5. The filtrate is FA 1. You are provided with: FA 1 saturated solution of magnesium carbonate FA 2 0.0300 mol dm-3 hydrochloric acid solution Methyl orange indicator (b) (i) Dilution of FA 2 The concentration of FA 2 is too high to be used in the titration. You will dilute FA 2 before using it in (c). 1. Pipette 25.0 cm3 of FA 2 into a 250 cm3 volumetric flask. 2. Top up to the mark with deionised water. 3. Stopper and shake thoroughly.

4 © ACJC 2019 9729/04/Prelim/2019 [Turn over (b) (ii) Calculate the concentration of hydrochloric acid in diluted FA 2. [HCl] in diluted FA 2 = ………………………………… [1] (c) (i) Titration of FA 1 against diluted FA 2 1. Fill the burette with diluted FA 2 from (b). 2. Use a pipette to transfer 10.0 cm3 of FA 1 into a 100 cm3 conical flask. 3. Add 1 – 2 drops of methyl orange indicator into the conical flask. 4. Run diluted FA 2 from the burette into this flask until the appropriate colour change is observed. 5. Record your titration results, to an appropriate level of precision, in the space provided. 6. Repeat steps 1 to 5 until consistent results are obtained. Results [3]

5 © ACJC 2019 9729/04/Prelim/2019 [Turn over (c) (ii) From your titrations, obtain a suitable volume of diluted FA 2, to be used in your calculations. Show clearly how you obtained this volume. Volume of diluted FA 2 = ………………………………… [3] (d) (i) Calculate the concentration of carbonate ions in FA 1, using your answers in (b)(ii) and (c)(ii). [CO32-] in FA 1 = ………………………………… [2] (ii) Hence, calculate the solubility product of magnesium carbonate. State its units. Ksp of magnesium carbonate = ……………………………………. units ………………………………… [1]

6 © ACJC 2019 9729/04/Prelim/2019 [Turn over (e) In (a), it was instructed that the filtrate had to be clear and that the residue should not be washed. (i) State and explain the effect on titre volume if the filtrate was not clear. effect on titre volume ………………………………………………………………… explanation …………………………………………………………………………… …………………………………………………………………………………………. ……………………………………………………………………………………… [1] (ii) Explain why the residue should not be washed. ………………………………………………………………………………….……… ……………………………………………………………………………….………… ……………………………………………………………………………………… [1] (f) The dissociation of magnesium carbonate in water is endothermic. Sketch a graph to show how the solubility product of magnesium carbonate varies with temperature. Explain your answer. explanation …………………………………………………………………………………… …………………………………………………………………………………………………. ……………………………………………………………………………………………… [2] Ksp temperature

7 © ACJC 2019 9729/04/Prelim/2019 [Turn over (g) Planning The technique of g ravimetry includes all analytical methods in which the analytical property is a measurement of mass or a change in mass. This technique can also be used in determining the solubility product of magnesium carbonate. One of the possible approaches is to determine the mass of precipitate formed after mixing two solutions of known c oncentration. After preparing a mixture of saturated solution containing the precipitate, it can be passed through a pre-weighed filter paper that retains the precipitate. Residual moisture can be removed by drying the filter paper and the precipitate. The amount of precipitate can be determined, from which the concentration of ions in the saturated solution and hence solubility product can be determined. (i) Plan an investigation to determine the solubility product of magnesium carbonate using the approach outlined above. You are provided with the following:  0.400 mol dm-3 sodium carbonate solution  0.400 mol dm-3 magnesium nitrate solution  apparatus and equipment normally found in a school laboratory In your plan you should include brief details of:  the apparatus you would use,  the quantities you would use,  the procedure you would follow,  the measurements you would make to allow for gravimetric analysis. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. ………………………………………………………………………………………….

8 © ACJC 2019 9729/04/Prelim/2019 [Turn over …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. ……………………………………………………………………………………… [4] (ii) Show how the measurements recorded in (g)(i) can be used to calculate the concentration of Mg2+ and CO32-, in mol dm-3, in the saturated solution. [2] [Total: 20]

9 © ACJC 2019 9729/04/Prelim/2019 [Turn over 2 Determination of the concentration of NaHCO 3 and the value for the enthalpy change of reaction of NaHCO3 and NaOH FA 3 is a solution of sodium hydrogencarbonate, NaHCO3, of unknown concentration. FA 4 is 1.50 mol dm–3 sodium hydroxide, NaOH. In this experiment, you will perform a thermometric titration to determine the equivalence point for the reaction of FA 3 and FA 4, where maximum heat is evolved, without the use of an indicator. You will follow the reaction by measuring the temperature as volumes of FA 4 are added in regular portions from a burette to a fixed volume of FA 3 placed in a Styrofoam cup. The data obtained will allow you to determin