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Chemistry · Practice book · Senior 6 · Unit 10 of 14

Solubility and solubility product of sparingly soluble salts

Senior 6 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 10 Solubility and solubility product of sparingly soluble salts

Key unit competence: Calculate the solubility product constant of sparingly soluble salts and deduce its applications.

Key ideas

  • Solubility is the maximum amount of solute that dissolves in a given amount of solvent at a given temperature; molar solubility s is in mol dm−3. A solution is unsaturated (can dissolve more), saturated (no more dissolves) or supersaturated (holds more than the saturated amount, unstable).
  • In a saturated solution of a sparingly soluble salt, a heterogeneous equilibrium exists: AgCl(s) ⇌ Ag+(aq) + Cl−(aq). The solubility product Ksp = [Ag+][Cl−] (the solid is left out). For PbI2: Ksp = [Pb2+][I−]2.
  • Relationship to solubility: AB: Ksp = s2; AB2 or A2B: Ksp = 4s3; AB3: 27s4. Only compare Ksp values directly for salts of the same formula type.
  • The ionic product Q has the same form as Ksp using actual concentrations: Q > Ksp: precipitate forms; Q = Ksp: saturated; Q < Ksp: no precipitate. In fractional precipitation the salt with the smaller solubility precipitates first (e.g. AgI before AgCl).
  • Common ion effect: a salt is less soluble in a solution that already contains one of its ions (AgCl in NaCl solution). pH: hydroxides and salts of weak acids (carbonates, sulfides) dissolve more in acid. Complex formation raises solubility: AgCl dissolves in ammonia as [Ag(NH3)2]+.
  • Applications: qualitative analysis, removing hardness and heavy metals from water, kidney stones (calcium oxalate), barium meal (BaSO4, so insoluble it is safe), tooth decay and fluoride (fluorapatite is less soluble in acid), precipitating salts in industry.

Common misconceptions

  • "A smaller Ksp always means a less soluble salt." No: Only for salts with the same formula type; for different types calculate s.
  • "Adding NaCl to saturated AgCl solution makes more AgCl dissolve." No: The common ion Cl− lowers the solubility of AgCl.
  • "A precipitate forms whenever two ions are mixed." No: It forms only if the ionic product exceeds Ksp.

Worked examples

Example 1. Calculate the mass of BaSO4 (M = 233) that dissolves in 250 cm3 of water (Ksp = 1.1 × 10−10).

Solution

s2 = 1.1 × 10−10, so s = 1.05 × 10−5 mol dm−3.

In 0.250 dm3: n = 1.05 × 10−5 × 0.250 = 2.62 × 10−6 mol.

Mass = 2.62 × 10−6 × 233 = 6.1 × 10−4 g (0.61 mg).

So a barium meal releases very few Ba2+ ions.

Example 2. A solution contains 0.010 mol dm−3 Cl− and 0.010 mol dm−3 I−. AgNO3 is added slowly. Ksp: AgCl 1.8 × 10−10, AgI 8.5 × 10−17. Which precipitates first and how well are they separated?

Solution

[Ag+] to start AgI: 8.5 × 10−17 ÷ 0.010 = 8.5 × 10−15.

[Ag+] to start AgCl: 1.8 × 10−10 ÷ 0.010 = 1.8 × 10−8.

AgI precipitates first. When AgCl starts, [I−] left = 8.5 × 10−17 ÷ 1.8 × 10−8 = 4.7 × 10−9 mol dm−3.

Almost all iodide (99.99995 %) is removed before AgCl forms: good separation (fractional precipitation).

Practice questions

  1. Define solubility product.

    Show answer

    The product of the concentrations of the ions in a saturated solution of a sparingly soluble salt, each raised to the power of its coefficient, at a given temperature.

    Common mistake: Including the solid in the expression.

  2. Write Ksp for Ca3(PO4)2.

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    [Ca2+]3[PO43−]2

    Common mistake: Writing [Ca2+][PO43−].

  3. Write Ksp in terms of s for Al(OH)3.

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    Ksp = s × (3s)3 = 27s4.

    Common mistake: Writing 3s4.

  4. What is the ionic product?

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    The product of ion concentrations (with powers) in any solution, not necessarily saturated.

    Common mistake: Saying it equals Ksp always.

  5. Explain the common ion effect.

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    Adding a solution containing one of the salt's ions shifts the solubility equilibrium towards the solid, lowering solubility.

    Common mistake: Saying it changes Ksp.

  6. Why does Mg(OH)2 dissolve in dilute HCl?

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    H+ reacts with OH−, lowering [OH−] so more solid dissolves.

    Common mistake: Saying HCl is a complexing agent.

  7. Give two applications of Ksp.

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    Qualitative analysis, water treatment (removing Ca2+/heavy metals), predicting precipitation, understanding kidney stones (any two).

    Common mistake: Listing 'making fertilisers'.

  8. Why is Ksp used only for sparingly soluble salts?

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    In concentrated solutions ions interact strongly, so the simple expression is no longer accurate.

    Common mistake: Saying soluble salts have no ions.

  9. How does a seed crystal affect a supersaturated solution?

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    It makes the excess solute crystallise until the solution is just saturated.

    Common mistake: Saying it dissolves.

  10. Why is AgCl more soluble in ammonia than in water?

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    Ag+ forms the complex [Ag(NH3)2]+, removing free Ag+.

    Common mistake: Saying ammonia is an acid.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. A precipitate forms when:a. H+ removes carbonate ions as CO2
    2. Ksp changes only with:b. the ionic product exceeds Ksp
    3. AgCl dissolves in aqueous ammonia because:c. temperature
    4. Calcium carbonate dissolves in acid rain because:d. a soluble complex [Ag(NH3)2]+ forms
    Show answer

    1 → b (the ionic product exceeds Ksp) · 2 → c (temperature) · 3 → d (a soluble complex [Ag(NH3)2]+ forms) · 4 → a (H+ removes carbonate ions as CO2)

    • Q > Ksp.
    • Like all equilibrium constants.
    • Complex formation removes Ag+.
    • Statues and limestone erode.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    s2  ·  mol3 dm−9  ·  mol dm−3  ·  mol2 dm−6
    1. For a salt AB, Ksp in terms of s is .
    2. The units of Ksp for AgCl are .
    3. Molar solubility is measured in .
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    1: s2 · 2: mol2 dm−6 · 3: mol dm−3

    • [A] = [B] = s.
    • (mol dm−3)2.
    • Moles dissolved per dm3 of saturated solution.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. For a salt AB2, Ksp in terms of s is:a. cannot dissolve any more solute at that temperature
    2. The units of Ksp for Mg(OH)2 are:b. 4s3
    3. Ksp for AgCl is:c. mol3 dm−9
    4. A saturated solution is one that:d. [Ag+][Cl−]
    Show answer

    1 → b (4s3) · 2 → c (mol3 dm−9) · 3 → d ([Ag+][Cl−]) · 4 → a (cannot dissolve any more solute at that temperature)

    • s × (2s)2.
    • (mol dm−3)3.
    • The solid is left out.
    • Equilibrium with undissolved solid.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    has no effect  ·  decreases the solubility of a salt  ·  H+ removes OH−, shifting the equilibrium  ·  [Pb2+][I−]2
    1. The common ion effect .
    2. Ksp for PbI2 is .
    3. Metal hydroxides are more soluble in acids because .
    Show answer

    1: decreases the solubility of a salt · 2: [Pb2+][I−]2 · 3: H+ removes OH−, shifting the equilibrium

    • Shifts the equilibrium to the solid.
    • Power = coefficient.
    • Le Châtelier.

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