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Equilibrium

Solubility Equilibria — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 10 parts Lesson 4 of 19 Unit quiz

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Solubility Equilibria — AP CHEMISTRY notes: Use Ksp to describe saturated solutions, calculate solubility, predict precipitation and explain the common-ion effect.Ksp — the equilibrium constant for dissolving a sparingly soluble ionic solidFor MgCl2(s) ⇌ Mg2+ + 2Cl−: Ksp = [Mg2+]…

  • Interpretation
  • Ion concentrations
  • Use of Ksp
  • Definition
  • Explanation
  • Calculation

Section 1 of 10Solubility product

Ksp
  • Ksp the equilibrium constant for dissolving a sparingly soluble ionic solid
  • For MgCl2(s) ⇌ Mg2+ + 2Cl−: Ksp = [Mg2+][Cl−]2.
  • The solid is omitted from the expression.
Saturated solution
  • Saturated solution ions leave and return to the solid at equal rates
Interpretation
  • Interpretation a smaller Ksp means lower solubility for salts of the same ion ratio

Section 2 of 10Calculating Ksp from solubility

Calculating Ksp from solubility
Write the dissolution equation and Ksp expression.
Use the ratio in the equation to find each ion concentration from the solubility s.
Substitute into Ksp.

Section 3 of 10Pattern: PbBr₂

Equation
  • PbBr2(s) ⇌ Pb2+ + 2Br−
Ion concentrations
  • [Pb2+] = s and [Br−] = 2s
Result
  • Ksp = s(2s)2 = 4s3 = 1.07 × 10-8 for s = 1.39 × 10-3 M

Section 4 of 10Calculating solubility from Ksp

CuO
  • 1:1 ratio, so Ksp = s2 and s = √Ksp
Use of Ksp
  • Use of Ksp valid only for sparingly soluble salts, not Group 1, nitrate or ammonium salts

Section 5 of 10Predicting precipitation

ComparisonMeaning
Q > KspPrecipitate forms
Q = KspSaturated, at equilibrium
Q < KspNo precipitate; more can dissolve

Section 6 of 10Mixing solutions

Dilution
  • Dilution equal volumes halve each concentration before Q is calculated
Example
  • 8.50 × 10-4 M Ca2+ and 7.50 × 10-4 M SO42− give Q = 6.4 × 10-7 < Ksp = 2.0 × 10-5, so no precipitate

Section 7 of 10Common-ion effect

Definition
  • Definition adding a soluble salt with an ion common to a sparingly soluble salt lowers its solubility
Explanation
  • Explanation the extra ion shifts the equilibrium toward the solid, as when KCl is added to saturated AgCl
Calculation
  • PbCl2 in 0.10 M NaCl has Ksp = x(0.10)2, so x = 1.7 × 10-3 M
  • In pure water it is 0.016 M, about ten times larger.

Section 8 of 10More from the specification

Molar solubility
  • Moles of salt dissolving per litre of saturated solution.
Chloride
  • Salt of hydrochloric acid.
Sulfate
  • Salt of sulfuric acid.
Compound
  • Two or more elements chemically combined in a fixed ratio.
Insoluble
  • Unable to dissolve.
Solubility product
  • The product of ion concentrations in a saturated solution, each to its coefficient.

Section 9 of 10Key terms

TermMeaning
Solubility product KspEquilibrium constant for dissolution of a sparingly soluble ionic solid.
Molar solubilityMoles of solute that dissolve per litre of saturated solution.
Saturated solutionSolution in equilibrium with undissolved solute.
Common-ion effectReduced solubility caused by an added ion common to the salt.
PrecipitateSolid formed when Q exceeds Ksp.

Section 10 of 10Exam technique

1
Putting the solid in the Ksp expression.
2
Forgetting to raise the ion concentration to its coefficient, for example [Br−]2.
3
Forgetting dilution when two solutions are mixed.
4
Comparing solubility of salts using Ksp when the ion ratios differ.

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