ACJC Prelim P2_QP

3 © ACJC 2018 9729/02/Prelim/2018 [Turn over Answer all the questions in the spaces provided. 1 A mixture contains two white solids XCO3 and YSO4 where X and Y could be either magnesium, zinc or barium. The following investigations were carried out to determine the identity of X and Y. (a) When the mixture of solids was treated with excess dilute hydrochloric acid, a colourless gas was evolved and some, but not all, of the mixture dissolved. The resulting mixture was filtered. To the filtrate, dilute aqueous ammonia was added dropwise till in excess. The white precipitate formed was insoluble in excess aqueous ammonia. Deduce the identity of elements X and Y, showing your reasoning clearly. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………. [3] (b) Group 2 carbonates decompose according to the following equation: ZCO 3(s) ZO(s) + CO2(g) element magnesium calcium barium enthalpy change for the reaction, ∆Hr / kJ mol−1 +101 +178 +269 (i) Use the data above to state and explain the trend in the thermal stability of the Group 2 carbonates. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. …………………………………………………………………………………………. ……………………………………………………………………………………… [3]

4 © ACJC 2018 9729/02/Prelim/2018 [Turn over (ii) Lithium carbonate, a Group 1 carbonate, decomposes on heating. Use relevant data from the Data booklet to calculate relative charge densities of the above cations and hence deduce which of the above ∆Hr values in the table is likely to be the nearest to that of lithium carbonate. [2] (c) Patients with digestive tract problems are sometimes asked to take an x-ray after they have swallowed a “barium meal”, consisting of a suspension of BaSO 4 in water. (i) Write an expression for the solubility product, Ksp for BaSO4, including its units. …………………………….………………………………………………………… [2] (ii) The numerical value of Ksp is 1.30 × 10–10. Calculate [Ba 2+(aq)] in a saturated solution of BaSO4. [1] (iii) Although Ba 2+ ions are toxic, the [Ba 2+(aq)] in a saturated solution of BaSO 4 is too low to cause any problems of toxicity. The numerical value of K sp for BaCO3 (5 × 10–10) is not significantly higher than that of BaSO4, but BaCO3 is very poisonous if inges