Colligative Formula

Question

what formula do I use for finding freezing points?

Answer 1

The formula you use for boiling point elevation or freezing point depression is

∆T = i m K

∆T = change in boiling or freezing point

i = van't Hoff factor

m = molality = moles solute / kg solvent

K = boiling or freezing point constant for the solvent.

To find i (van't Hoff factor) you have to know the formula of the solute. If a non-electrolyte, i = 1. If an electrolyte that ionizes or dissociates, it is the number of particles that it provides. For example, for NaCl, i = 2, for CaCl₂, i = 3; for AlCl₃, i = 4, etc.

To find m (molality), you divide the moles of solute by the kg of solvent.

To find the new boiling point, you solve for ∆T and add that value to the normal boiling point of the solvent.

To find the new freezing point, you solve for ∆T and subtract that value for the normal freezing point.

Answer 2

Colligative properties depend on the number of solute particles present in a solution rather than the nature of the solute itself. One of the colligative properties is the depression of the freezing point of a solution. To find the new freezing point of a solution, you can use the freezing point depression formula:

ΔTf = Kf · m · i

where:

- ΔTf is the change in the freezing point temperature (the difference between the freezing point of the pure solvent and the freezing point of the solution),

- Kf is the freezing point depression constant or cryoscopic constant, which is a property of the solvent (different solvents have different Kf values),

- m is the molality of the solution (moles of solute per kilogram of solvent),

- i is the van't Hoff factor, which represents the number of particles a solute molecule dissociates into in the solution (for non-electrolytes like sugar, i = 1; for electrolytes like NaCl, i > 1, as it dissociates into multiple ions).

To find the new freezing point of the solution, subtract the calculated freezing point depression (ΔTf) from the freezing point of the pure solvent:

New freezing point = (Freezing point of pure solvent) - ΔTf