Molar Mass Determination from freezing point

Question

If I have calculated the freezing point using the cooling curve on excel (took the line of best fits from the initial temperature and plateau and set equal to each-other and solved). How can I now calculate the molecular weight of a unknown with this information, the Kf( given as 4.49), and the initial and final temperature. 1g of the unknown was put into 5 grams pure steric acid. The equation given is MW= Kf x (m solute/m solvent x Δ Tf). With the information given, am I able to just plug in the information I have? Do I need to use the freezing point or do I use the change in temperature? 

Answer 1

If you know the ∆T, Kf and the van't Hoff factor (i), you can solve for the molality (m) in the equation

∆T = imK

Once you know the molality, you can solve for the molar mass of the unknown as follows:

m = moles/kg and you have 0.005 g (5 g), so solve for moles.

Then 1 g unknown / moles unknown = molar mass unknown

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