A solution containing 0.750 g of an unknown substance in 30.0 g of cyclohexane was found to freeze at 3.1 ◦C. What is the molar mass of the unknown substance?

Colligative property is a phenomenon where some physical properties of solvent (chemically pure substances i.e. cyclohexane) become disturbed as a result of adding other substances (usually a solute in this case).

Some of the colligative properties include:

The depression of  a freezing point can be computed by using the formula:

where;

At standard condition for cyclohexane:

Given that after adding the non-volatile unknown substance:

∴

Using the formula for the depression of freezing point, the molality of the unknown substance can be estimated as:

molality (m) = 0.17 mol/kg

Using the relation of molality =

Making molar mass the subject of the formula and substituting the values, we have:

The molar mass of the unknown substance = 147.06 g/mol

Therefore, we can conclude that the molar mass of the unknown substance is 147.06 g/mol

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