An aqueous solution containing 35.9 g of an unknown molecular (nonelectrolyte) compound in 150.0 g of water has a freezing point of −1.3 °C. Calculate the molar mass of the unknown compound.
Solution: Here, we are going to calculate the molar mass of the unknown compound.
The freezing point of a substance may be defined as the temperature at which the vapour pressure of the substance in its liquid phase is equal to its vapour pressure in the solid phase.
In order to determine the molar mass of the solute, te following expression can be used:
Kf x w2 x 1000
M2 = ------------------------ -----(1)
ΔTf x w1
Where, M2 = molar mass of the solute
Kf = freezing point depression constant
w2 = given mass of solute in grams
ΔTf = depression in freezing point
w1 = mass of solvent in grams