In isotonicity calculations, which method uses the freezing point depression equation?

• A. Freezing Point Depression
• B. Osmolality measurement
• C. Boiling Point Elevation
• D. Vapor Pressure Lowering

Answer: (A)

In isotonicity calculations, you use a colligative-property approach, and the freezing point depression method specifically relies on the equation ΔTf = iKf m. This equation tells you how much a solute lowers the solvent’s freezing point based on the number of particles (via i, the van’t Hoff factor) and the molality (m). By measuring how many degrees the solution’s freezing point drops, you can calculate its molality and then convert to osmolarity to match body fluids, aiming for about 290 mOsm/kg. That direct link between freezing point change and particle concentration is why the freezing point depression method is used for isotonicity calculations. The other options involve different colligative properties or direct osmolarity measurement, which don’t use this freezing-point relationship.