Question

An aqueous solution containing 36.2 g of an unknown molecular (non-electrolyte) compound in 146.1 g of water was found to have a freezing point of -1.2 ∘C. Calculate the molar mass of the unknown compound.

Answer 1

Weiight of solute = 36.2 g

Amount of water = 146.1 g

So, in 1000g or 1 Kg of water, 247 g solute will be present.

K_b for water = 0.512 ^oC Kg/mol

Freezing point of water = 0.0 ^oC

Freezing point of solution = -1.2 ^oC

Change in temperature, T = 1.2 ^oC

Suppose, M = molar mass of solute, m = molality

Thus, T = K_b/m => m = T/K_b = 0.512/1.2 = 0.4266 mol/g

=> 247/M = 0.4266    

=> M = 247/0.4266 = 578.99 g/mol