Question

Calculate the molar mass of an unknown solid (assume a non-electrolyte), if 1.35 grams of th esolid was dissolved in 15.0 grams of dodecanol and caused a freezing point depression of 2.34

Answer 1

Mass of solid dissolved m1 = 1.35 g
Mass of dodecanol m2 = 15 g
Freezing point depression Td= 2.34 C
Freezing point depression constant for dodecanol K = 2.9 C/mol
Calculating the molarity of dodecanol m => Td = K x molarity
=> 2.34 = 2.9 x molarity => molarity = 2.34 / 2.9
Molarity M = 0.807
We have mass and molarity, calculating for molarity for 15 g solvent
= 15 x (0.806 / 1000) = 0.0121
Now calculating the molar mass of solid = m1 / molarity
=> 1.35 / 0.0121 = 111.538
So the molar mass of the Solid is 111.54