An aqueous solution containing 17.5 g of an unknown molecular (nonelectrolyte) compound in 100.0 g of water has a freezing point of −1.8 °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
Freezing point of water = 0oC
Freezing point of the solution = -1.8oC
Therefore, depression in freezing point(ΔTf) = 0oC - (-1.8oC)