Colligative Properties and Molar Mass Calculations

Identify which property is mentioned: boiling point, freezing point, vapour pressure, or osmosis. Choose the matching formula. Convert temperature to kelvin difference where needed, volume to litres or cubic metres consistently, and solvent mass to kilograms for molality. Substitute values, then state whether the result is property change, molar mass, or osmotic pressure.

A phase-change comparison sketch may help show elevation in boiling point and depression in freezing point, but numerical formula selection is the central skill.

Using molarity in Delta T_b = K_b m or Delta T_f = K_f m is a common error because these formulas require molality.

Colligative properties are solution properties that depend on the number of solute particles present, not on their chemical nature, for dilute solutions.

Colligative properties are solution properties that depend on the number of solute particles present, not on their chemical nature, for dilute solutions. Elevation of boiling point: Delta T_b = K_b m. Depression of freezing point: Delta T_f = K_f m. Osmotic pressure: pi = C R T = nRT/V. Relative lowering of vapour pressure: (p^0 - p)/p^0 = x_solute for ideal dilute solutions. Adding salt to water increases the boiling point and lowers the freezing point because the number of solute particles affects phase-change conditions.

The four main colligative properties are relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, and osmotic pressure. They are used to calculate molar mass of solutes and to compare particle numbers in solution. For electrolytes or associating solutes, the observed effect may differ because the number of particles changes. Elevation of boiling point: Delta T_b = K_b m. Depression of freezing point: Delta T_f = K_f m. Osmotic pressure: pi = C R T = nRT/V. Relative lowering of vapour pressure: (p^0 - p)/p^0 = x_solute for ideal dilute solutions. A solution has molality 0.20 mol kg^-1 and K_f = 1.86 K kg mol^-1. Delta T_f = K_f m = 1.86 x 0.20 = 0.372 K. Interpretation: the freezing point is lowered by 0.372 K from the pure solvent value. Questions usually ask calculation of Delta T_b, Delta T_f, osmotic pressure, molar mass, or comparison of colligative effect for different solutes. Long-answer questions may combine formula selection with abnormal molar mass. Using molarity in Delta T_b = K_b m or Delta T_f = K_f m is a common error because these formulas require molality.