Question

How do you calculate freezing point from molality?

Answer 1

Hint: The number moles of the solute present in one kg of solvent are called molality and it is going to be denoted with a symbol ‘m’.
The temperature at which the liquid will convert to solid is called the freezing point of that particular compound.

Complete step by step answer:
- In the question it is asked how we can calculate the freezing point from molality.
- There is a relationship between molality of the solution and freezing point of the compound and it is as follows.
\[\Delta {{T}_{f}} = im{{K}_{f}}\]
Here $\Delta {{T}_{f}}$ = change in freezing point of the solution
i = Van’t Hoff factor
m = molality of the solution
${{K}_{f}}$ = depression in freezing point of the solution.
- To get the freezing point of the solution we should know the change in freezing point of the solution to get the new freezing point of the solution.
New freezing point of the solution = normal freezing point of the solution - $\Delta {{T}_{f}}$ .
- The formula to calculate the molality of the solution is as follows.
\[\text{molality=}\dfrac{\text{number of moles of the solute}}{\text{volume of solvent in kg}}\]

Note: Van’t Hoff factor of a solute can be calculated based on the number of ions that are going to be generated by a particular solute when it is dissolved in water. For example van’t Hoff factor (i) for calcium chloride ($CaC{{l}_{2}}$ ) is three because it is going to generate one calcium and two chloride ions when dissolved in water. Then the van’t Hoff factor for calcium chloride is three.