MeOH:H2O Freezing Point Depression Calculation Issue

[Voltux]

I noticed that my windshield washer fluid has methanol in it to reduce the temperature of which it freezes. I know it freezes at approximately -28°C which gives us a concentration of MeOH at approximately ~30% by weight.

However, when I try to calculate the required methanol using the mSolute = dT/Kf I get 15.054 mol/kg which is 482.5g/L of MeOH. That turns out to be about 609mL/L if you convert that to volume with the density of 0.792g/mL.

Can someone explain where I went wrong in my calculations, or how to properly calculate this?

Data:
Kf Constant for Water: 1.86
MeOH Molar Mass: 32.05g/mol
MeOH Density: 0.792g/mL
Temperature Difference: 28°

 

[Borek]

First of all, you are using an invalid model.

Freezing point depression with Kf assumes the solute to be nonvolatile, that's definitely not a case with the methanol.