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. [3]

4 (iii) Describe and explain how the solubility of Zn(OH)2 is affected: • by adding HCl(aq) • by adding ZnCl2(aq). [2] Zn(OH)2(s) ⇌ Zn2+(aq) + 2OH–(aq) 4. 2024/P2/Q5d A sample of sea water contains 0.0050 mol dm–3 of Ca2+ (aq) and 0.0480 mol dm–3 of Mg2+ (aq). The Ksp of the metal hydroxides of these ions is shown in the table below. Ca2+ Mg2+ Ksp for metal hydroxides at 298K / mol3 dm–9 5.5 x 10–6 1.1 x 10–11 (i) Use your knowledge of solubility products and the information in the table above to explain why Ca2+ ions in this sample remain in solution at pH 12 but the Mg2+ ions do not. [2] (ii) Show by relevant calculations that almost all the Mg2+(aq) present in this sample forms a precipitate at pH 12. [2]

5 5. 2012/P2/Q3(d) Selective precipitation of sulfides may be used to separate the cations present in a solution containing Cu2+, Ni2+ and Zn2+. The reagent used is hydrogen sulfide which behaves as a dibasic (diprotic) weak acid when in aqueous solution. and H2S(g) + aq H+(aq) + HS–(aq) HS–(aq) + aq H+(aq) + S2–(aq) Ka = 9.1 × 10–8 mol dm–3 Ka = 1.0 × 10–15 mol dm–3 Metal sulfides are precipitated by the following reaction. M2+(aq) + S2–(aq) → MS(s) Relevant Ksp values are given in the table. Salt Ksp / mol2 dm–6 CuS 6.3 × 10–36 NiS 4.0 × 10–21 ZnS 1.6 × 10–24 The results of passing hydrogen sulfide through separate solutions of Cu 2+, Ni2+, and Zn2+ are shown in the table below. M2+ being tested H2S passed through an acidic solution of M2+ H2S passed through a neutral solution of M2+ H2S passed through an alkaline solution of M2+ Cu2+ black precipitate of CuS black precipitate of CuS black precipitate of CuS Ni2+ no precipitate no precipitate black precipitate of NiS Zn2+ no precipitate white precipitate of ZnS white precipitate of ZnS Use the data from above to give an explanation for each of the following questions. You should give equations where appropriate. You are not expected to do any calculations in this part. (i) Why is CuS the only sulfide of these three precipitated in acid solution? [2]

6 (ii) Why is NiS precipitated in alkaline solution and not in neutral or acidic solution? [2] 6. 2019/P2/Q5g