Question

Calculate the molar mass of a nonelectrolyte compound if a 15.0 g of the compound dissolved in 150 g of CCl4 has a boiling point that is 2.50°C higher than the solvent. The boiling point constant, kb, is 5.03°C/molal.

Answer 1

Elevation in boiling point can be written as

∆T_b = iK_bm

∆T_b = boiling point elevation = 2.50°C

i = vant hoff factor = 1 for non electrolyte

K_b = molal elevation constant = 5.03°C/molal

m = molality

m = ∆T_b/K_b = 2.50°C/5.03°C/molal = 0.50 molal or 0.50 moles/kg

now molality = moles of solute/mass of solvent in kg

mass of solvent CCl₄ = 150g =0.15 kg

0.50 mol/kg = moles of solute(n)/0.15 kg

moles of solute(n) = 0.075 mol

moles = mass(m)/molar mass(M)

mass given = 15.0g

0.075 mol = 15.0g/M

M = 200 g/mol