# Solubility Product Constant, Ksp

The Solubility Product Constant, Ksp is the equilibrium constant for a solid substance dissolving in an aqueous solution. It represents the level at which a solute dissolves in solution. A more a substance dissolves, the higher the Ksp value it has.

Example:

$aA (s) \rightleftharpoons cC (aq) + dD (aq)$

To solve for the Ksp it is necessary to take the molarities or concentrations of the products (cC and dD) and multiply them. If there are coefficients in front of any of the products, it is necessary to raise the product to that coefficient power(and also multiply the concentration by that coefficient). This is shown here:

$K_{sp} = [C]^c [D]^d$

Please note that the reactant, aA, is not included in the Ksp equation. Solids are not included when calculating equilibrium constant expressions, since their concentration does not change the expression; any change in their concentration is insignificant therefore left out.

Ksp represents the maximum amount of solid that can be dissolved in the aqueous solution. The answer will have the units of mol/L, molarity, or better known as concentration.

### Some things to note

• For highly soluble ionic compounds you have to find the ionic activities versus the concentrations that are found in slightly soluble solutions.
• Common Ion Effect: The solubility of the reaction is reduced by the common ion, so if dealing with two of the same reactions the reaction with the ion will have a lesser Ksp and the reaction without the ion will have a greater Ksp.
• Salt Effect (diverse ion effect): Having an opposing effect on the Ksp value compared to the common ion effect, uncommon ions increase the Ksp value. Uncommon ions are those that are different from those involved in equilibrium.
• Ion Pairs: When you have an ionic pair (cation and anion) the Ksp value calculated will be lower than the actual value due to ions involved in pairing. To reach the calculated Ksp value, more solute must be added.

Try these!

• What is the solubility product constant expression for MgF2?

MgF2(s) ↔ Mg2+(aq) + 2F-(aq) so the Ksp = [Mg2+][F-]2

• What is the solubility product constant expression for Ag2CrO4?

Ag2CrO4(s) ↔ 2Ag+(aq) + CrO42-(aq) so the Ksp = [Ag]+2[CrO42-]

### References

1. Petrucci, Ralph H., et al. General Chemistry: Principles and Modern Applications. Upper Saddle River, NJ: Prentice Hall 2007.

### Contributors

• Kathryn Rashe, Lisa Peterson

22:48, 5 Oct 2013

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