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) Ca