Question

0.680 g of a compound is dissolved in 15.0 g of benzene, and the freezing point of the solution is lowered by 1.44°C.

Calculate the experimental molecular mass of the compound.

(K_f for benzene = 5.12 K kg mol⁻¹)

Answer 1

Given: Mass of solute (w₂) = 0.680 g

Mass of solvent (benzene, w₁) = 15.0 g 

= 0.015 kg

Freezing point depression (ΔT_f) = 1.44 K (or °C)

Cryoscopic constant (K_f) for benzene = 5.12 Kg mol⁻¹

Formula: ΔT_f = K_f × m

m = `(ΔT_f)/(K_f)`

= `1.44/5.12`

= 0.28125 mol/kg

Moles = m × kg solvent

= 0.28125 × 0.015

= 0.00421875 mol

Molar mass = `0.680/0.00421875`

= 161 g/mol