Question

Explain why pure liquids and solids can be ignored while writing the equilibrium constant expression?

Answer 1

For a pure substance (both solids and liquids),

$[\text {Pure\ Substance}] = \frac {\text{Number \ of\ moles}} {\text{Volume}}$

$= \frac {\frac {\text {Mass}}{\text {molecular \ mass}}}{\text {Volume}}$

$= \frac {\text {Mass}}{\text {Volume × Molecular }mass}$

$= \frac {\text {Density}}{\text {Molecular\ mass}}$

Now, the molecular mass and density (at a particular temperature) of a pure substance is always fixed and is accounted for in the equilibrium constant. Therefore, the values of pure substances are not mentioned in the equilibrium constant expression.