if Kf for water is 1.86 geg mol-1, a 0.1m solution of NaCl in water will have freezing point of
What's a 1.86 geg mol-1?
delta T = i*Kf*m
i = 2 for NaCl.
Substitute the numbers and solve for delta T, then subtract that from zero C to find the freezing point of the solution.