Under what condition is the expression π = i M R T valid for osmotic pressure?

• A. For dilute solutions that behave ideally.
• B. For concentrated solutions with strong interactions.
• C. Only at temperatures near absolute zero.
• D. Only for non-electrolyte solutes.

Answer: (A)

Osmotic pressure follows an ideal, particle-counting behavior when the solution is dilute, so the number of solute particles controls the pressure just as in an ideal gas. In that regime the derivation leads to π = i M R T, where M is the molar concentration, T is temperature in kelvin, R is the gas constant, and i is the factor that counts how many particles appear per formula unit in solution (accounting for dissociation of electrolytes). For dilute solutions with negligible interactions between solute molecules, activity corrections are small and the simple proportionality holds. If the solution becomes concentrated, solute–solute and solute–solvent interactions cause deviations from this relationship, so the equation no longer strictly applies. The statement isn’t limited to non-electrolytes; i adjusts for electrolytes as well, so the essential condition is ideal, dilute behavior, not a restriction to a specific solute type.