Question

A 0.756 g sample of an unknown nonelectrolyte is dissolved in 11.451 g of benzophenone and produces a solution that freezes at 45.2 degrees C. If pure benzophenone melted at 48.0 degrees C, what is the molecular weight of the compound? I am familiar with the formula but I am confused with whether to use molality or molarity and how to calculate it.

Answer 1

Tf   = i*Kf *m

i = 1 for non electrolyte

Kf   = 9.8⁰C/m

Tf    = 48-45.2   = 2.8⁰C

Tf   = i*Kf *m

2.8     = 1*9.8*m

m      = 2.8/9.8   = 0.286m

molality   = W*1000/G.M.Wt * weight of solvent in g

0.286       = 0.756*1000/G.M.Wt*11.451

G.M.Wt    = 0.756*1000/0.286*11.451   = 230.84g/mole

molar mass of unknown   = 230.84g/mole