For the concentration of pure solid or pure liquid, Molar conc. = $\frac{\text{Moles of the substance}}{\text{Volume of the substance}}$ = $\frac{\frac{\text{Mass}}{\text{Molar mass}}}{\text{Volume}}$ = $\frac{\text{Mass}}{\text{Volume}}$ × $\frac{1}{\text{Molar mass}}$ = $\frac{\text{Denisty}}{\text{Molecular mass}}$ Since density of pure solid or liquid is constant at constant temperature and molar mass is also constant therefore, their molar concentrations are constant and are included in the equilibrium constant.