ion product
Summary
The ion product is the product of dissolved ion concentrations, each raised to its coefficient. For water it is Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C. For a sparingly soluble salt it is Q, compared with Ksp to predict whether a precipitate forms.
Detail
Meaning: The ion product is the product of dissolved ion concentrations, each raised to the power of its coefficient. For water, 2H₂O ⇌ H₃O⁺ + OH⁻ gives Kw = 1.0 × 10⁻¹⁴ at 25 °C, so pH + pOH = 14; Kw rises with temperature. For a sparingly soluble salt, the ion product Q uses the actual concentrations in the mixture and is compared with Ksp.
Equation: Q = [A]ˣ[B]ʸ for the salt AₓBᵧ, where [A] and [B] are ion concentrations (mol/L) and x and y are the coefficients in the dissolution equation; Q = Ksp at saturation. For water, Kw = [H⁺][OH⁻].
On the MCAT: Two solutions are mixed, such as Pb²⁺ and Cl⁻. Find Q after dilution and compare it with Ksp. Q > Ksp means a precipitate forms until Q falls to Ksp; Q < Ksp means the solution is unsaturated and nothing precipitates; Q = Ksp means it is saturated. Adding a common ion raises Q and lowers solubility.
Common confusion: Ksp is the ion product only at saturation equilibrium, while Q can have any value. Students also forget that mixing dilutes each ion, and forget to raise each concentration to its coefficient.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- General Chemistry (Petrucci)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.