Answer to Question #298433 in General Chemistry for Hyb d

Question

A solution prepared by adding 10g of an unknown non-volatile, non-electrolyte solute to 100 g of water. The Boiling point of the solution is elevated by 0.433 0C above the normal boiling point of the solvent. What is the molar mass of the unknown substance?

Answer 1

∆T_b = i x K_b x m

∆T_b =T_{b solution} - T_{b pure} = change in boiling point

K_b = boiling point elevation constant

i = Vant Hoff factor of solute

m = molality = mol solute / kg solvent

∆T_b = 0.433 °C

K_b = 0.51 °C/m

For non-volatile, non-electrolyte solute i = 1

m = ∆T_b / (i x K_b) = 0.433 °C / (1 x 0.51 °C/m) = 0.849 mol/kg

m = 10 g / (Mr(solute) x 0.1 kg) = 0.849 mol/kg

Mr(solute) = 10 g / (0.849 mol/kg x 0.1 kg) = 117.8 g/mol