Osmolality measurement by cryoscopic methods is considered a colligative property because it depends on which factor?

• A. temperature at equilibrium is a function of particle number
• B. particle type determines osmolality
• C. solution color determines osmolality
• D. particle size determines osmolality

Answer: (A)

Osmolality by cryoscopic methods is a colligative property because the effect depends on how many solute particles are present, not on what those particles are. In freezing-point depression, used in cryoscopy, the change in temperature at which the solution freezes is proportional to the number of dissolved particles (and, for electrolytes, the dissociation factor, i). So more particles mean a larger depression and a higher calculated osmolality. The identity of the particles—whether they’re glucose, sodium chloride, or another solute—doesn’t matter for this relationship, nor do their color or size.