Question

An aqueous salt solution is formed by adding 11.67 g sodium sulfate (solute) to water (solvent). What mass (in g) of water is used if the freezing point of the solution is -12.9 oF.

Kf H2O = 1.86 oC/m

I know to use delta T= i*Kf*m
I then solve for m. But, then I'm confused on what numbers to plug into the other parts of the equation. I've tried 3 different answers and don't know what I'm doing wrong or how far I'm off.

Answer 1

moles Na2SO4 = grams/molar mass.
Solve for moles Na2SO4.

Convert -12.9F to C.
delta T = i*Kf*m
i = 3, you know Kf and dT, solve for m

m = moles Na2SO4 from above/kg solvent.
Solve for kg solvent.