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The freezing point of ethanol is -114.6 C and its Kf value is 2.00 C/m. What is the freezing point for the solution prepared by dissolving 50.0 g of glycerin (C3H8O3) in 200.0 g ethanol?

I used Change in temp=Kfm
I calculated m to be 2.1736.
I ended up the answer of -118.4 C but the answer is apparently -120. I'm a few numbers off, can someone explain to me where I went wrong?

  • Chemistry -

    mols glycerin = grams/molar mass = 50/92 = about 0.54 but you can clean that up (as well as the numbers following also).

    m = mols/kg solvent
    m = 0.54/0.200 = about 2.7

    dT = Kf*m = 2.00*2.7 = about 5.4.

    New freezing point = -114.6-5.4 = about 120.

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