In an experiment, 3.545 grams of an unknown nonelectrolyte is dissolved in 15.0 grams of water. The extrapolated freezing point is -.56 degrees C. What is the molecular weight of the unknown material?

Answer 1

delta T = Kf*molality
Solve for molality.

m = moles/kg solvent
Solve for moles. You have m and 0.15 kg solvent)

moles = grams/molar mass
You have moles and grams, solve for molar mass.

Answer 2

Ok so when I solved for molality I got 0.30108. Then I plugged in and solved for moles and I got .0045 moles. So when I solved for molar mass I got 784.96 g/mol. Does that sound right? Thank you so much!!

Answer 3

That looks ok to me. I think you are allowed only three significant figures so I would round to 785.