Question

How does molality affect the freezing point?

Answer 1

The freezing point and the molality of the solution is related with each other by the colligative property known as depression in freezing point. The molality is the number of moles of solute dissolved in one kilogram of solvent.

Complete step by step answer:
The relation between the molality and the freezing point is known by the depression in freezing point.
The depression in freezing point is one of the colligative properties. The colligative property is defined as the property of the solution which is dependent on the concentration of the solute particles and not on the nature of the solute particles.
The freezing point depression is defined as the lowering of the freezing point of the solvent by the addition of the solutes which can be explained using Raoult’s law. According to Raoul's law when a nonvolatile solute is added in the solvent, the vapour pressure of the solvent decreases by the addition of solute.
The freezing point is defined as the point at which any substance present in the liquid state starts to freeze.
The depression in freezing point is given by the formula as shown below.
$\Delta {T_f} = i \times {K_f} \times m$
Where,
$\Delta {T_f}$ is the depression in freezing point.
i is the Van’t Hoff factor
${K_f}$ is the cryoscopic constant
m is the molality
The depression in freezing point is directly proportional to the molality of the solution where the molality is the number of solute dissolved in one kilogram of the solvent.
When, the $T_f^0$ is the freezing point of the pure solvent, ${T_f}$ is the freezing point of the solution and $\Delta {T_f}$ is the depression in freezing point.
So, the freezing point of the solution will be given as
$\Delta {T_f} = T_f^0 - {T_f}$
So, when more concentrated will be the solution in other words more number of solute particles, the freezing point will be depressed more.

Note: The formula of the depression in freezing point is also used to determine the molecular weight of the compound by using the formula of molality. To calculate the molecular weight the mass should be known.