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MeOH:H2O Freezing Point Depression Calculation Issue

  1. Jan 3, 2017 #1
    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?

    Kf Constant for Water: 1.86
    MeOH Molar Mass: 32.05g/mol
    MeOH Density: 0.792g/mL
    Temperature Difference: 28°
  2. jcsd
  3. Jan 3, 2017 #2


    User Avatar

    Staff: Mentor

    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.
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