Information - Ionizing vs Nonionizing Solutes
The freezing point of a solvent can be depressed by the presence of a dissolved substance. The extent of the freezing point depression, ΔTf, of a solvent is a colligative property - one which does not depend on identity of the solute but rather the effective number of particles each dissolved molecule releases into the solution. An ionizing molecule will separates into smaller cations and anions in solution such that the solution remains neutral. A nonionizing molecule does not separate into smaller particles when it dissolved in a solvent to form a solution. This effective number of particles, i, is called the van 't Hoff factor, it is unitless.
Problems
1. For the ionizing molecule NaCl, estimate its van 't Hoff factor to the nearest integer value.
2. For the nonionizing molecule urea, estimate its van 't Hoff factor to the nearest integer value.
Information - Extent of Freezing Point Depression
Because the freezing point depression is a colligative property, it depends only upon the amount of solute dissolved, the van 't Hoff factor of that solute, and the freezing point depression constant of the solvent. The extent to which the freezing point of a solvent will be depressed by the addition of a solute is calculated with:
ΔTf = iKfm (1)
where ΔTf is the amount by which the freezing point will be depressed, i is the van 't Hoff factor, Kf is the freezing point depression constant of the solvent, and m is the molality of the solute in the solvent (mol solute/kg solvent). For water Kf = 1.86 °C kg/mol.
Problems
3. If an aqueous NaCl solution is 1.00 m, what is the expected freezing point temperature of that solution?
4. Write an equation that relates the solution freezing point, Tf, soln, the solvent freezing point, Tf, solv, and the freezing point depression, ΔTf.