SAJC 2026 Solubility Equilibria Discussion (Student)

1 St Andrew’s Junior College H2 Chemistry 2025 Tutorial 20: Solubility Equilibria DISCUSSION QUESTIONS 1. 2018/P3/Q1b Buildings and statues constructed of limestone (calcium carbonate, CaCO 3) can be corroded by acid rain. They can partially protected by painting them with a solution containing ammonium N-phenyloxamate. ammonium N-phenyloxamate The ammonium salt reacts with the surface of the limestone to produce an inert layer of calcium N-phenyloxamate. The by-product, ammonium carbonate, is water soluble. (i) Write an equation for the reaction between calcium carbonate and ammonium N - phenyloxamate, NH4(C8H6NO3). [1] (ii) The solubility product, Ksp, f or calcium N -phenyloxamate is 1.75 x 10 –5 mol3 dm–9. Calculate [calcium N-phenyloxamate] in a saturated solution in g dm–3. [3]

2 2. ACJC 2009/P3/Q4(a) Hard water is water that has higher calcium and magnesium content. Using hard water results in sediments accumulating in electrical appliances such as water heaters and washing machines. This raises the electricity consumption rate and shortens the lifespan of the appliances. Hard water is treated through a softening process, where the metal ions are removed, before it is used for consumption. A sample of hard water containing Mg 2+(aq) was collected. The presence of Mg 2+ is to be tested by precipitating MgCO3. (a) Write an expression for the solubility product, Ksp, of magnesium hydroxide. [1] (b) The solubility of Mg(OH) 2 in pure water at 25 oC is 9.08 x 10 –3 g dm –3. For a saturated aqueous solution of Mg(OH)2 at 25 oC, calculate the concentrations of Mg2+ and OH– and prove that the solubility product, Ksp, of Mg(OH)2 is 1.5 x 10–11 mol3 dm–9. [2] (c) How would you expect the solubility of magnesium hydroxide in aqueous magnesium nitrate to compare with that in pure water? Discuss this effect using Le Chatelier’s Principle. [2]

3 (d) The concentration of Mg2+ in the water sample was found to be 3.2 x 10–4 mol dm–3. A 50 cm 3 sample was used in this test. To this sample, 30 cm 3 of a 5.0 x 10 –4 mol dm –3 aqueous sodium hydroxide was added. Using the Ksp value in (b), deduce if a precipitate would appear when the two solutions were added. [2] 3. 2020/P3/Q5c Zinc hydroxide, Zn(OH)2, is sparingly soluble in water. Zn(OH)2(s) ⇌ Zn2+(aq) + 2OH–(aq) The numerical value of Ksp is 2.0 × 10–17 at 25 oC. (i) Write an expression for the Ksp of Zn(OH)2 stating its units. [1] (ii) Calculate the pH of a saturated aqueous solution of Zn(OH)2 at 25 oC