Question

An aqueous solution containing 36.3 g of an unknown molecular (non-electrolyte) compound in 144.2 g of water was found to have a freezing point of -1.2 ∘C.Calculate the molar mass of the unknown compound.

Answer 1

delta Tf = Kf * molality

where, delta Tf = depression in freezing point

delta Tf = freezing point of pure solvent - freezing point of solution

delta Tf = 0- (-1.2) = 1.2 ⁰C

1.2 ⁰C = 1.86 * molality

molality = 1.2 / 1.86 = 0.645 m

molality = number of moles of solute / mass of solution in kg

0.645 = number of moles of solute / 0.1442 kg

number of moles of solute = 0.645 * 0.1442 = 0.0930 mole

number of moles of solute = mass / molar mass

0.0930 mole = 36.3 g / molar mass of solute

molar mass of solute = 36.3 g / 0.0930 mole = 390 g/mol

Therefore, the molar mass of solute = 390 g/mol