KEY2CHEM

Solubility Equilibrium

 

The dissolution of a ionic compound (or salt), such as \(\text{NaCl}\), is a reversible process:

\(\require{mhchem}\ce{NaCl(s) <=> Na+(aq) + Cl^{-}(aq)}\)

Where the dissolving of the solid ionic compound is in equilibrium with the recrystallizing of the dissolved ions to re-form the solid. The equilibrium constant for this reaction is the solubility product constant, \(K_{sp}\). The larger the \(K_{sp}\) value, the more solid dissolves into its ions (meaning, the more soluble the salt is).

 


Example 1.

 

Which salt is predicted to have the lowest solubility in pure water? \(K_{sp}\) values are provided.

 

A. \(\require{mhchem}\ce{BaF2}; K_{sp} = 1.8 \times 10^{-7}\)

B. \(\require{mhchem}\ce{MgF2}; K_{sp} = 7.4 \times 10^{-11}\)

C. \(\require{mhchem}\ce{SrF2}; K_{sp} = 2.5 \times 10^{-9}\)

 

 

 

 

Solution

 

 

B. \(\require{mhchem}\ce{MgF2}; K_{sp} = 7.4 \times 10^{-11}\)

The salt with the smallest solubility product constant (\(K_{sp}\)) has the lowest concentration of dissolved ions at equilibrium and the lowest solubility.

 


Example 2.

Place the following salts in order of increasing solubility in pure water. \(K_{sp} \) values are provided.

 

\(\require{mhchem}\ce{PbCl2}; K_{sp} = 1.2 \times 10^{-5}\)

\(\require{mhchem}\ce{PbBr2}; K_{sp} = 6.6 \times 10^{-6}\)

\(\require{mhchem}\ce{PbI2}; K_{sp} = 8.5 \times 10^{-9}\)

 

A. \(\require{mhchem}\ce{PbCl2} < \require{mhchem}\ce{PbBr2} < \require{mhchem}\ce{PbI2}\)

B. \(\require{mhchem}\ce{PbI2} < \require{mhchem}\ce{PbBr2} < \require{mhchem}\ce{PbCl2}\)

C. \(\require{mhchem}\ce{PbBr2} < \require{mhchem}\ce{PbCl2} < \require{mhchem}\ce{PbI2}\)

 

Solution

 

 

B. \( \require{mhchem}\ce{PbI2} < \require{mhchem}\ce{PbBr2} < \require{mhchem}\ce{PbCl2}\)

The salt with the smallest solubility product constant (\(K_{sp}\)) has the lowest concentration of dissolved ions at equilibrium and the lowest solubility. The salt with the largest \(K_{sp}\) value has the highest concentration of dissolved ions at equilibrium and the greatest solubility.


Example 3.

Which salt is most soluble in pure water? \(K_{sp} \) values are provided.

A. \(\require{mhchem}\ce{NaCl}; K_{sp} = 36\)

B. \(\require{mhchem}\ce{AgCl}; K_{sp} = 1.8 \times 10^{-10}\)

C. \(\require{mhchem}\ce{CuCl}; K_{sp} = 1.7 \times 10^{-7}\)

 

 

 

Solution

A. \(\require{mhchem}\ce{NaCl}; K_{sp} = 36\)

The salt with the largest \(K_{sp}\) value has the highest concentration of dissolved ions at equilibrium and the greatest solubility.