colligative properties, properties of a solution that depend on the number of solute particles present but not on the chemical properties of the solute. Colligative properties of a solution include freezing point (see freezing), boiling point, osmotic pressure (see osmosis), and solvent vapor pressure. By measuring these properties and comparing them with the corresponding properties of the pure solvent, it is possible to determine the number of particles of solute present in the solution. If the mass of solute present is also known, the number-average molecular weight can be calculated by dividing the mass of solute by the number of particles present to obtain the average mass per particle.