SAJC 2026 Solubility Equilibria Self Check (Student)

1 St Andrew’s Junior College H2 Chemistry 2026 Tutorial 20: Solubility Equilibria SELF–CHECK QUESTIONS 1. Write an expression for Ksp of the following sparingly soluble salts , and calculate the unknown solubility products and solubilities in mol dm –3. Salt Ksp expression with units Ksp value Solubility / mol dm–3 CaCO3 Ksp= [Ca2+][CO32–] Units = mol2 dm–6 4.8 x 10–9 6.93 x 10–5 ZnS Ksp= [Zn2+][S2–] Units = mol2 dm–6 1.6 x 10–23 4.00 x 10–12 Cu2O Ksp= [Cu+]2[O2–] Units = mol3 dm–9 2.11 x 10–15 8.08 x 10–6 Fe2S3 Ksp= [Fe3+]2[S2–]3 Units = mol5 dm–15 1.0 x 10–88 9.85 x 10–19 Worked example for Cu2O Cu2O(s) ⇌ 2Cu+(aq) + O2–(aq) In 1 dm3 of solution, 8.08 x 10–6 mol of Cu2O dissolves, giving • [Cu+] = 2 x 8.08 x 10–6 mol dm–3 • [O2-] = 8.08 x 10–6 mol dm–3 Ksp = [Cu+]2[O2–] = [2 x 8.08 x 10–6]2[8.08 x 10–6] = 2.11 x 10–15 mol dm–3. Worked example for Fe2S3 Fe2S3(s) ⇌ 2Fe3+(aq) + 3S2–(aq) Let the solubility be z mol dm –3. In 1 dm 3 of solution, z mol of Fe2S3 dissolves, giving • [Fe3+] = 2z mol dm–3 • [S2-] = 3z mol dm–3 Ksp = [Fe3+]2[S2–]3 1.0 x 10–88 = (2z)2(3z)3 = 36z5 z = 9.85 x 10 –19 mol dm–3

2 2. [2017/P1/Q15] 3. [2008/P2/Q2] The solubility of barium hydroxide, Ba(OH)2 at 25 oC is 0.24 g dm–3. (a) (i) Calculate the concentration of the saturated aqueous solution, in terms of mol dm–3. [1] (ii) If it is assumed that the solute is completely ionised, calculate the hydroxide ion concentration of the solution. [1] (b) (i) Write an expression for the solubility product of barium hydroxide. [1] (ii) Using the values obtained in part (a), calculate the value of the solubility product, stating the units. [1] (c) Bottles containing aqueous barium hydroxide need to be kept firmly stoppered or white deposit forms on the surface. (i) What is the white deposit? [1] (ii) Explain, with the aid of an equation, how it is formed. [2]

3 4. [2011/P3/Q3b] One of the most common phosphorous–containing rocks is fluorapatite, Ca5(PO4)3F. The first step in the production of phosphate as a mineral supplement for animal feed is defluorination, which is achieved by heating the rock in a rotary kiln at 1500 oC with silica and steam. The following equation represents this process. 6Ca5(PO4)3F + aSiO2 + bH2O → cSiF4 + dHF + eCaO + fCa3(PO4)2 (i) Complete the balancing of the above equation by deducing values for a, b, c, d, e and f. [3] (ii) Write the expression for the solubility product of calcium phosphate, stating its units. [2] (iii) The value of Ksp for calcium phosphate is 1 × 10–26. Calculate [Ca2+] in a saturated solution of calcium phosphate. Show your working clearly. [1] 5. A solution A is saturated with both calcium hydroxide and calcium sulfate. Both salts are sparingly soluble. (a) The pH of solution A is