Question

Determine the molar mass of a compound and its molecular formula if a solution of 1.00 g of the unknown is dissolved in 50 grams of cyclohexane. Cyclohexane freezes at 6.02 oC. The normal freezing point of cyclohexane is 6.50 oC and its kf value is 20.2 oC/m. The percent composition of the unknown is 85.71% C and 14.29% H

Answer 1

delta T = Kf*m
Substitute and solve for m = molality.

m = mols/kg solvent
You know m and kg solvent, solve for mols.

mols = g/molar mass.
You know mols and g, solve for molar mass. I obtained in the neighborhood of 842 as the approximate molar mass.

For the compound with 85.71%C and 14.29%H, convert to mols.
85.71/12 = about 7.14 mols C
14.29g/1 = 14.29 mols H which is a ratio of CH2. That's the empirical formula with an empirical mass of 14
842/14 = about 60.2 which rounds to 60 so the molecular formula is (CH2)₆₀ or C60H120 with a more accurate molar mass of (60*12 + 120*1) = ?