Question

1. Calculate the molar mass of an unknown solid (assume a non-electrolyte), if 1.35 grams of the solid was dissolved in 15. grams of dodecanol and caused a freezing point depression of 2.34 degrees C. the freezing point constant, k_f_ is 2.9 degree C.

2. (use information from #1) Calculate the molar mass of this compound if it were actually an ionic compound that dissociates into two parts (i.e. BX->B^+ +X^-).

freezing point of dodecanol is 27.5 C

Answer 1

1)

freezing point depression = 2.34 oC

Kf = 2.9

delta Tf = Kf x m

2.34 = 2.9 x m

m = 0.807 m

molality = 0.807 m

moles of solute / mass of solvent in kg = 0.807

n / 0.015 = 0.807

n = 0.0121

mass / molar mass = 0.0121

1.35 / M = 0.0121

molar mass = 111.5 g/mol

2)

delta Tf = i x Kf x m

2.34 = 2 x 2.9 x m

m = 0.403 m

molality = 0.403 m

moles of solute / mass of solvent in kg = 0.403

n / 0.015 = 0.403

n = 6.05 x 10^-3

mass / molar mass = 6.05 x 10^-3

1.35 / M =6.05 x 10^-3

molar mass = 223 g/mol