When 0.73 g of an unknown non-electrolyte is dissolved in 50.0 g of benzene, the freezing point decreased to 5.22 degrees C from 5.5 degrees C.

Question

If the K_fp of the solvent is 5.12 K/m, calculate the molar mass of the unknown solute.

Answer 1

∆T = imK

∆T = change in freezing point = 5.5 - 5.22 = 0.28 degrees

i = van't Hoff factor = 1 for a non electrolyte

m = molality = moles of unknown/kg solvent = ?

K = constant = 5.12

Solving for m, we have...

m = ∆T/(i)(K) = 0.28/5.12 = 0.0547 m = 0.0547 moles unknown/kg solvent

Since the mass of solvent is 50 g that is = 0.050 kg

0.0547 mol/kg x 0.050 kg = 0.00274 moles

molar mass = 0.73 g/0.00274 mol = 266 g/mol