colligative properties

Meaning: Colligative properties depend on the number of dissolved solute particles, not their identity. The four tested are vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. Solute lowers the solvent's tendency to escape, so vapor pressure falls, boiling point rises, and freezing point falls. The simple formulas assume a dilute, ideal solution.

Equation: ΔTf = i Kf m (and ΔTb = i Kb m); π = iMRT. Here ΔT is the change in temperature (K), i is the van 't Hoff factor (unitless), Kf and Kb are solvent constants (K·kg/mol), m is molality (mol/kg), M is molarity (mol/m³ in SI), R is the gas constant (J/(mol·K)), T is temperature (K), and π is osmotic pressure (Pa).

On the MCAT: Rank solutions by i × concentration, such as 0.1 m glucose, NaCl, and CaCl₂ (ideal i = 1, 2, 3). Other setups include salt melting ice, cells shrinking or swelling in hypertonic or hypotonic solutions, and phase diagrams with shifted boundaries.

Common confusion: Equal molality does not mean equal effect. Ionic solutes dissociate, so ideally 1 m NaCl lowers the freezing point about twice as much as 1 m glucose.