Question

Calculate the molar mass of a substance if the addition of 48.50 g of the substance to 400.0 g of ethyl acetate (K_b=2.86 degrees celsius/ molality) elevated the boiling point of the ethyl acetate by 2.05 degrees celsius.

Answer 1

Given that mass of substance added w = 48.5 g

molar mass of substance added M = ?

mass of solvent ( ethyl acetate ) W = 400 g = 0.4 kg

             Kb = 2.86 ^oC/m

elevation in the boiling point dT =  2.05 ^oC

We know that

   elevation in the boiling point ΔT = Kf x molality

molality = (w/M) x (1/W in kg)

Then,   elevation in the boiling point ΔT = Kf x { (w/M) x (1/W) }

M = (Kf x w ) / (ΔT x W)

=  (2.86 ^oC/m x 48.5 g) / (2.05 ^oC x  0.4 kg)

= 169.1 g/mol

M = 169.1 g/mol

Therefore, molar mass of substance added = 169.1 g/mol