SAJC 2026 Solubility Equilibria Lecture Notes (Student)

St. Andrew’s Junior College JC2 H2 Chemistry 2026 1 St Andrew’s Junior College H2 Chemistry 2026 Lecture Notes 20 Solubility Equilibria Assessment Objectives: Candidates should be able to: (a) show understanding of, and apply, the concept of solubility product, Ksp (b) calculate Ksp from concentrations and vice versa. (c) discuss the effects on the solubility of ionic salts by the following: (i) common ion effect (ii) formation of complex ion, as exemplified by the reactions of halide ions with aqueous silver ions followed by aqueous ammonia (see also an Introduction to the Chemistry of Transition Elements) Lecture Outline 1. Solubility and Solubility Product 2. Precipitation and Ionic Product 3. Factors affecting solubility SLS Resources (req @students.edu.sg login) Lesson 1: Solubility and Solubility Product Lesson 2: Precipitation of Sparingly Soluble Salts Lesson 3: Common Ion Effect https://vle.learning.moe.edu.sg/moe- library/module/view/99023736-991d- 4605-a652-df3a6f984c25 https://vle.learning.moe.edu.sg/moe- library/module/view/ed3826a7-ce8c- 4140-97eb-fb189c6f077a https://vle.learning.moe.edu.sg/moe- library/module/view/c29629d4-141c- 4284-8f43-367950bf8405

St. Andrew’s Junior College JC2 H2 Chemistry 2026 2 1. SOLUBILITY AND SOLUBILITY PRODUCT Salts can be classified as soluble or insoluble. But even “insoluble” salts dissolve to a very small extent in water. A saturated solution is a solution in which the maximum amount of solute/salt has been dissolved. A saturated solution of an insoluble salt usually has a concentration of less than 0.001 mol dm –3. Contrast this to a saturated solution of sodium chloride, which has a concentration of 6.15 mol dm–3 For this chapter on solubility equilibrium, we will use the term “sparingly soluble” to describe insoluble salts. Soluble Sparingly soluble All nitrates NIL Most halides Halides of Pb2+, Ag+ and Cu+ Most sulfates Sulfates of Ba2+, Pb2+ and Ca2+ Oxides and hydroxides of Na+, K+, NH4+ and the larger group 2 cations such as Ca2+, Sr2+ and Ba2+ Most oxides and hydroxides Carbonates of Na+, K+ and NH4+ Most carbonates Chromates of Na+, K+ and NH4+ Most chromates Table 1: Soluble and sparingly soluble salts (no need to memorise!!!!) 1.1 Solubility product When a sparingly soluble salt e.g. AgCl solid is added slowly to pure water, it initially dissolves, but will eventually reach a point when no further solid dissolves and undissolved solid is seen. At this point, a saturated solution is formed. The ions in the saturated solution are in dynamic equilibrium with the excess undissolved solid. This means that the ions move from the solid to the saturated solution at the same rate as they move from the solution to the solid. AgCl(s) ⇌ Ag+(aq) + Cl–(aq) We can write an equilibrium constant for this equilibrium. Kc = [Ag+(aq)][Cl–(aq)] Recall from chemical equilibria that the concentration of a pure solid and pure liquid is constant and is omitted from the Kc expression.

St. Andrew’s Junior College JC2 H2 Chemistry 2026 3 The equilibrium constant for the dissolving of a sparingly soluble salt in water is known as its solubility product, Ksp. Hence, Ksp = [Ag+(aq)][Cl–(aq)] units: mol2 dm–6 (state symbols not req) The solubility product of a sparingly soluble salt is the product of the concentrations of its constituent ions in a saturated solution, each raised to the power of its stoichiometric coefficient in the balanced equation. Similar to Kc and Kp, the value of Ksp is constant at a constant temperature, and its units depends on the stoichiometric coefficients of the relevant ions. For a sparingly soluble salt with general formula, AxBy AxBy(s) ⇌ xAy+(aq) + yBx–(aq) Ksp = [Ay+(aq)]x[Bx–(aq)]y Units = (mol dm–3)x+y Solute Ksp Solute Ksp AgCl 1.8 x 10–10 mol2 dm–6 Zn(OH)2 2.0 x 10–17 mol3 dm–9 BaSO4 1.3 x 10–10 mol2 dm–6 Fe(OH)2 7.9 x 10–16 mol3 dm–9 CaCO3 4.8 x 10–9 mol2 dm–6 Mg(OH)2 1.1 x 10–11 mol3 dm–9 ZnS 1.6 x 10–23 mol2 dm–6 Fe2S3 1.0 x 10–88 mol5 dm–15 Table 2: Some solubility products of sparingly soluble salts in pure water at 25 oC (no need to memorise!!!!) Exercise 1 Write expressions for the Ksp and state its units: (i) Ag2CO3 (ii) Ca3(PO4)2 Ag2CO3(s) ⇌ 2Ag+(aq) + CO32–(aq) Ksp = [Ag+]2 [CO32–] units: mol3 dm–9 Ca3(PO4)2(s) ⇌ 3Ca2+(aq) + 2PO43–(aq) Ksp = [Ca2+]3 [PO43–]2 units: mol5 dm–15

St. Andrew’s Junior College JC2 H2 Chemistry 2026 4 1.2 Solubility The solubility of a sparingly soluble salt is the maximum amount of solute that can dissolve in 1 dm3 of solution. The solubility of a sparingly soluble salt indicates the concentration of its constituent ions in a saturated solution at a given temperature (assume 298K unless otherwise stated). Solubility can be expressed in g dm–3, or more commonly, in mol dm–3. E.g. The solubility of AgCl is 1.34 x 10–5 mol dm–3 AgCl(s) ⇌ Ag+(aq) + Cl–(aq) In 1 dm3 of solution, 1.34 x 10–5 mol of AgCl dissolves, giving • [Ag+] = 1.34 x 10–5 mol dm–3 • [Cl–] = 1.34 x 10–5 mol dm–3 Solubility is not the same as solubility product Ksp! Solubility Ksp solubility product • Maximum amount of salt that can be dissolved in 1 dm3 of solution • Value can change depending on solvent e.g. dissolve in H2SO4 vs H2O • Units in g dm-3 or mol dm-3 • An equilibrium constant • Constant at constant temperature • Units depends on formula of salt 1.3 Converting between solubility product and solubility, or vice versa You may be asked to calculate the solubility product of a compound from its solubility, or vice versa.

St. Andrew’s Junior College JC2 H2 Chemistry 2026 5 Exercise 2 The solubility of Fe(OH)2 is 5.8 x 10–6 mol dm–3. Calculate its solubility product, stating its units. Fe(OH)2(s) ⇌ Fe2+(aq) + 2OH–(aq) In 1 dm3 of solution, 5.8 x 10–6 mol of Fe(OH)2 dissolves, giving • [Fe2+] = 5.8 x 10–6 mol dm–3 • [OH–] = 2 x 5.8 x 10–6 = 1.16 x 10–5 mol dm–3 Ksp = [Fe2+][OH–]2 = (5.8 x 10–6)(1.16 x 10–5)2 = 7.80 x 10–16 mol3 dm–9 Exercise 3 The solubility product of silver chromate, Ag2CrO4, is 1.1 x 10–12 mol3 dm–9 at 25oC. Calculate the solubility of silver chromate at 25oC. Ag2CrO4(s) ⇌ 2Ag+(aq) + CrO42–(aq) Let solubility of Ag2CrO4 be s mol dm–3 In 1 dm3 of solution, s mol of Ag2CrO4 dissolves, giving • [Ag+] = 2s mol dm–3 • [CrO42–] = s mol dm–3 Ksp = [Ag+]2[CrO42–] 1.1 x 10–12 = (2s)2(s) 1.1 x 10–12 = 4s3 s = 6.50 x 10–5 mol dm–3  Self–Check: Q1 – 2

St. Andrew’s Junior College JC2 H2 Chemistry 2026 6 1.4 Relationship between Solubility and Solubility Product Does a larger Ksp mean a higher solubility? • We can directly relate Ksp to solubility only if the salts have the same formula type e.g. AgCl, BaCO3 have the same formula type, with cation and anion in a 1:1 ratio e.g. CaF2, Zn(OH)2 have the same formula type, with cation and anion in a 1:2 ratio • If the formula type of the salt is different, the Ksp values have different units, and we cannot directly relate Ksp to solubility! Instead, calculation of solubility from given Ksp is required. Exercise 4 Calculate the solubilities of BaCO3 and CaF2. Which is the most and least soluble salt? Solute Ksp Solubility AgCl 1.8 x 10–10 mol2 dm–6 1.34 x 10–5 mol dm–3 BaCO3 5.0 x 10–9 mol2 dm–6 7.07 x 10–5 mol dm–3 CaF2 3.9 x 10–11 mol3 dm–9 2.14 x 10–4 mol dm–