determine the molar mass of an unknown nonvolatile solute of non electrolyte compound raising the boiling point of 150g of benzene by 0.500 degree Celsius

Question

This topic is about Calculating Boiling point elevation

Answer 1

Colligative properties:

Boiling point elevation...

∆T = imK

∆T = change in the boiling point = 0.500 degrees

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

m = molal = moles solute/kg solvent = ?

K = boiling point constant for benzene = 2.65º/m (had to look this up since it wasn't provided in the question)

We will solve for m (moles/kg) and since we know there are 0.150 kg (150 g), we an then find moles. We won't be able to find molar mass because we weren't told how many grams of unknown were used. But if you do have that information, you can find molar mass.

Solving for m, we have...

m = ∆T / (i)(K) = 0.5º / (1)(2.65º/m) = 0.189 m = 0.189 moles/kg

0.189 mols/kg x 0.150 kg = 0.0284 moles

If you know the grams used, the molar mass = grams/moles = g/0.0284 = molar mass