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March 27, 2017

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Urea (NH2)2CO is dissolved in 100.0 g of water. The solution freezes at -0.085 degrees C. how many grams of urea were dissolved to make this solution.

delta T = Kf*molality
Plug in 0.085 for delta T, I assume you know Kf for water, calculate molality.
Then molality x kg solvent x molar mass = grams.
You have all but molar mass which can be calculated.

0.085 degrees C = 0 degrees C (0.1 kg/60 g of (NH2)2CO

is this right

  • Chemistry - ,

    No.
    You haven't used Kf, the freezing point constant for water. You just ignored it in the calculation.

  • Chemistry - ,

    0 is the freezing point of water isnt it

  • Chemistry - ,

    wait no its 1.858 Kf(degress C/m)

  • Chemistry - ,

    right. Kf is 1.86 C/m.

  • Chemistry - ,

    so it would be 0.085 degrees C = 1.86 C/m (0.1 kg/60 g of (NH2)2CO

  • Chemistry - ,

    No. You haven't followed the first response I typed.
    0.085 = Kf*m
    Plug in Kf and solve for m.
    Then molality = mols/kg.
    You know molality and kg, solve for mols.
    Then mols = grams/molar mass. You know mols and you know molar mass, solve for grams.

  • Chemistry - ,

    DrBob222 needs to explain this better

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