Osmotic pressure is the pressure required to stop osmosis. For a dilute solution, which expression correctly gives osmotic pressure?

• A. π = i M R T
• B. π = M R T
• C. π = i M R T^2
• D. π = i M R / T

Answer: (A)

Osmotic pressure in a dilute solution follows the van't Hoff relation, which mirrors ideal-gas behavior: the pressure needed to stop osmosis is proportional to the solute concentration and to temperature. The correct form is π = i M R T, where M is molarity, R is the gas constant, T is absolute temperature, and i is the van't Hoff factor that accounts for how many particles the solute produces in solution (i = 1 for non-electrolytes, greater than 1 when dissociation occurs, like NaCl giving two particles). This linear dependence on T and on the effective number of solute particles explains why osmotic pressure increases with temperature and with solute dissociation. Expressions that omit i miss the effect of dissociation, while those with T^2 or 1/T do not match the observed linear dependence on temperature.