Many ionic compounds dissolve only slightly in water, so a saturated solution contains both dissolved ions and undissolved solid at the same time. The solubility product, Ksp, is the equilibrium constant that describes how many ions are present in that saturated solution. It matters because Ksp helps predict whether a precipitate forms and how much of a solid can dissolve.
Molar solubility connects this equilibrium idea to a measurable concentration in mol/L.
Key Facts
- For AgCl(s) ⇌ Ag+(aq) + Cl-(aq), Ksp = [Ag+][Cl-].
- Pure solids are not included in the Ksp expression because their activity is constant.
- Molar solubility, s, is the number of moles of solid that dissolve per liter of saturated solution.
- For CaF2(s) ⇌ Ca2+(aq) + 2F-(aq), Ksp = [Ca2+][F-]^2 = s(2s)^2 = 4s^3.
- Ion product Q = product of ion concentrations before equilibrium; if Q > Ksp, precipitation occurs.
- A common ion lowers molar solubility because Le Châtelier's principle shifts the dissolution equilibrium toward solid.
Vocabulary
- Ksp
- The solubility product constant is the equilibrium constant for a slightly soluble ionic solid dissolving into its ions.
- Molar solubility
- Molar solubility is the concentration of dissolved solid in a saturated solution, usually measured in mol/L.
- Saturated solution
- A saturated solution contains the maximum amount of dissolved solute possible at a given temperature while undissolved solid remains.
- Common-ion effect
- The common-ion effect is the decrease in solubility caused by adding an ion already present in the dissolution equilibrium.
- Ion product
- The ion product, Q, is the current product of dissolved ion concentrations used to predict whether precipitation will occur.
Common Mistakes to Avoid
- Including the solid in the Ksp expression is wrong because pure solids do not appear in equilibrium constant expressions.
- Forgetting stoichiometric coefficients is wrong because ions may form in unequal amounts, such as [F-] = 2s for CaF2.
- Assuming Ksp equals molar solubility is wrong because Ksp depends on the ion concentrations raised to powers from the balanced equation.
- Thinking a common ion changes Ksp is wrong because Ksp depends only on temperature, while the common ion changes the equilibrium concentrations and solubility.
Practice Questions
- 1 AgCl has Ksp = 1.8 x 10^-10 at 25°C. For AgCl(s) ⇌ Ag+(aq) + Cl-(aq), calculate the molar solubility in pure water.
- 2 The molar solubility of CaF2 in pure water is 2.1 x 10^-4 M. For CaF2(s) ⇌ Ca2+(aq) + 2F-(aq), calculate Ksp.
- 3 Explain why AgCl is less soluble in 0.10 M NaCl than in pure water, and state whether Ksp changes.