In order to find , the osmotic pressure, we consider equilibrium between a solution containing solute and pure water.
:

We can write the left hand side as:
:,where is the activity coefficient of the solvent. The product is also known as the activity of the solvent, which for water is the water activity . The addition to the pressure is expressed through the expression for the energy of expansion:
:where is the molar volume (m³/mol). Inserting the expression presented above