VJC Prelim P3_ANS

1  VJC 2015 9647/03/PRELIM/15 [Turn over Victoria Junior College 2015 H2 Chemistry Prelim Exam 9647/3 Suggested Answers 1 Magnesium ethanoate is commonly used as a source of magnesium or ethanoate ions in chemistry experiments. One of the more prevalent uses of magnesium ethanoate is in the mix ture called calcium magnesium ethanoate (CMA). It is a mixture of calcium ethanoate and magnesium ethanoate. CMA acts as a powerful SO 2, NO x, and toxic particulate emission control agent in coal combustion processes to reduce acid rain, and as an effective catalyst for the facilitation of coal combustion. (a) Samples of anhydrous magnesium ethanoate, Mg(CH 3COO)2 and barium ethanoa te, Ba(CH3COO)2, were heated to a temperature of 600 oC causing them to thermally decompose. White residues were formed in both cases and a common gaseous product, X, with molecular formula of C3H6O was also given off. X gives a yellow precipitate upon warming with aqueous alkaline iodine. In addition, the decomposition of magnesium ethanoate also produced a second gaseous product which formed a white precipitate with calcium hydroxide solution. (i) Identify the gaseous product, X. Propanone, CH3COCH3 [1] (ii) Write a balanced equation with state symbols, for each decomposition reaction. Mg(CH3COO)2 (s) MgO(s) + CO2(g) + CH3COCH3(g) Ba(CH3COO)2 (s) BaCO3(s) + CH3COCH3(g) [2] (iii) Account for the difference in the decomposition products. The Mg2+ ion is much smaller than Ba2+ ion, hence its higher charge density enables it to polarise the CH3COO- ion and distort the C-O bond to a greater extent resulting in complete decomposition. [2]

2  VJC 2015 9647/03/PRELIM/15 [Turn over (b) Magnesium, is an extremely important light wei ght structural metal which can be produced by the electrolysis of magnesium chloride . Magnesium chloride can be prepared from magnesium oxide w hich is obtained from sea -water containing a significant amount of Mg2+ and Ca2+. The steps involved are shown below: The numerical values of the relevant solubility products are given below. Magnesium carbonate 1.0 x 10-5 Calcium carbonate 8.7 x 10-9 Magnesium hydroxide 1.1 x 10-11 Calcium hydroxide 5.5 x 10-6 (i) Calculate and compare the solubility between magnesium carbonate and magnesium hydroxide. Ksp (Magnesium carbonate) = [Mg2+][CO3 2-] 1.0 x 10-5 = (x)(x) x = 3.16 x 10-3 mol dm-3 Ksp (Magnesium hydroxide) = [Mg2+][OH-]2 1.1 x 10-11 = (y) (2y)2 y = 1.40 x 10-4 mol dm-3 Solubility of MgCO3 is higher than that of Mg(OH)2 [3] (ii) Assuming the concentration of magnesium ions in the filtrate in step 2 is 3.0 x 10 -5 mol dm-3, calculate the concentration of hydroxide ions present in the filtrate