A solution containing 15.2 g of a molecular compound dissolved in 100.0 g of water has a boiling point of 101.5 °C. Calculate the molar mass of the compound. The 𝐾b for water is 0.512 °C/m.

Question

molar mass = g/mol?

Answer 1

Since the unknown is a molecular compound, we can assume it will not ionize/dissociate, and thus is a non electrolyte. It will have a van't Hoff factor of 1.

The formula to calculate change in boiling point is ∆T = imK

∆T = change in boiling point = 101.5º - 100º = 1.5º

i = van't Hoff factor = 1

m = molality = mols solute / kg water = ?

K = boiling constant = 0.512º/m

Solving for m, we have...

m = ∆T / iK = 1.5º / (1)(0.512º/m)

m = 0.988 mols / kg water

There are 100.0 g of water = 0.1000 kg water, so we can find the moles of unknown:

0.988 mols / kg x 0.1000 kg = 0.0988 mols

Molar mass = grams / moles

molar mass = 15.2 g / 0.0988 mols

molar mass = 154 g/mol