The normal freezing point of cyclohexane is 6.55 C. When 0.458 g of benzophenone is dissolved in 15.0 g of cyclohexane, the freezing point is found to be 3.19 C. What is the experimental molar mass of benzophenone? (Kf cyclohexane = 20.0 C m^-1).

A) 182
B) 866
C) 397
D) 191

(units are all g/mol)

i've tried to use the T2-T1=Kfm and solved for molality and then tried to use that to get grams of solute and molar mass equals moles/mass. but i don't get any of the above answers. So, i'm pretty sure i'm going about it the wrong way. what's the right answer and what steps should i do to get there ?

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  1. delta T = Kf*m
    Solve for m

    m = mols/kg solvent
    Solve for mols.

    mols = grams/molar mass.
    Solve for molar mass.

    I ran through it very fast and I obtained one of the answers.

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