2019 ACJC Prelim H2 Chem P4 ANS

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. Conc = 0.025 x 0.03 / 0.25 = 0.00300 mol dm-3 [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 Final burette reading / cm3 17.00 34.00 Initial burette reading / cm3 0.00 17.00 Vol of diluted FA 2 used / cm3 17.00 17.00 [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. (17.00 + 17.00) / 2 = 17.00 cm3 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). Amount of H+ = ans in (b)(ii) x ans in (c)(ii)/1000 Amount of CO32- = amount of H+ / 2 Concentration of CO32- = Amount of CO32- / 0.0100 = 0.5 [ans in (b)(ii) x ans in (c)(ii)/1000] / 0.0100 [CO32-] in FA 1 = ………………………………… [2] (ii) Hence, calculate the solubility product of magnesium carbonate. State its units. Ksp = [Mg2+][CO32-] = [ans from (d)(i)]^2 Units = mol2 dm-6 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 Titre volume will be larger explanation The residual carbonate will react with the HCl added [1] (ii) Explain why the residue should not be washed. Washing the residue will disturb the equilibrium and cause the solution to be below saturation point (and the titration results will be lower than actual, leading to lower than expected Ksp). [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 As dissociation is endothermic, increase in temperature cause POE of MgCO3(s) ↔ Mg 2+(aq) + CO 32-(aq) to shift right to favour reaction that absorbs heat, increasing concentration of ions, increasing Ksp OR When temperature increases, there is greater kinetic energy for the ions to break away from the ionic lattice, hence increasing concentration of ions, increasing Ksp OR Since reaction is endothermic, and entropy change is positive, when temperature is higher, reaction is more spontaneous, hence more MgCO 3 dissolves. [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 concentration. 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.  Measure 50.00 cm3 of sodium carbonate solution and 50.00 cm3 magnesium nitrate solution using two separate 50.00 cm3 burettes into a 250 cm3 beaker (and place the beaker in a temperature controlled water bath)  Stir the mixture and leave to stand for some time.  Filter the mixture using a pre-weighed filter paper (M1 g)  Dry precipitated MgCO3 together with filter paper  Weigh the dried precipitated MgCO3 together with the filter paper (M2 g). [4]

8 © ACJC 2019 9729/04/Prelim/2019 [Turn over (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. Amount of ppt = (M2-M1)/ Mr of MgCO3 = amount of Mg2+ not dissolved = amount of CO32- not dissolved Therefore, amount of Mg2+ dissolved = 0.4 x 50/1000 - (M2-M1)/Mr of MgCO3 amount of CO32- dissolved = 0.4 x 50/1000 - (M2-M1)/Mr of MgCO3 concentration of Mg2+ = [0.4 x 50/1000 - (M2-M1)/ Mr of MgCO3 ]/100/1000 concentration of CO32- = [0.4 x 50/1000 - (M2-M1)/ Mr of MgCO3 ]/100/1000 *dep