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Solubility Product Constant, Ksp

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  • The solubility product constant, \(K_{sp}\)​, is the equilibrium constant for a solid substance dissolving in an aqueous solution. It represents the level at which a solute dissolves in solution. The more soluble a substance is, the higher the \(K_{sp}\) value it has.

    Consider the general dissolution reaction below (in aqueous solutions):

    \[ aA_{(s)} \rightleftharpoons cC_{(aq)} + dD_{(aq)} \tag{1} \]

    To solve for the \(K_{sp}\) 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 below:

    \[ K_{sp} = [C]^c [D]^d \tag{2}\]

    Note that the reactant, aA, is not included in the \(K_{sp}\) equation. Solids are not included when calculating equilibrium constant expressions, because their concentrations do not change the expression; any change in their concentrations are insignificant, and therefore omitted. Hence, \(K_{sp}\) represents the maximum extent that a solid that can dissolved in solution.