A solution of 7.50 g of a nonvolatile molecular compound in 22.6 g of water boils at 100.78°C. What is the molar mass of the solute? (kb = 0.52°C/m)

Answer 1

Colligative property:

Boiling point elevation

∆T = imK

∆T = change in boiling point = 0.78º

i = van't Hoff factor = 1 for a nonvolatile molecular compound b/c it does not ionize or dissociate

m = molal = moles solute/kg solvent = ?

K = 0.52º/m

solving for m, we have...

m = ∆T / (i)(I) = 0.78 / (1)(0.52)

m = 1.5 = 1.5 moles / kg solvent

Molar mass = gram/mol so we need to find moles b/c we already know grams (7.50 g)

1.5 moles/kg x 0.0226 kg = 0.0339 moles

Molar mass = 1.50 g / 0.0339 moles = 44.2 g/mol