In the osmotic pressure equation π = i M R T for dilute solutions, what does i represent?

• A. The molar mass of solute.
• B. The universal gas constant.
• C. The van't Hoff factor, the effective number of particles in solution.
• D. The temperature in Kelvin.

Answer: (C)

Osmotic pressure depends on how many solute particles are present in the solution. The i in the equation is the van't Hoff factor, the effective number of particles produced when the solute dissolves. This factor adjusts the concentration term to reflect how the solute contributes to osmotic pressure.

For a nonelectrolyte that does not ionize, i is about 1, so π scales with M, R, and T just as in the simple gas-like analogy. For electrolytes that dissociate into ions, i reflects how many particles appear after dissociation (for a fully dissociated salt that gives two particles per formula unit, i ≈ 2). In real solutions, i can be a bit less than the ideal integer due to incomplete dissociation or ion pairing, but it still represents the effective particle count driving osmotic pressure.

The other quantities in the formula—M, R, and T—represent molarity, the gas constant, and temperature, respectively, and are not what i stands for.