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Calculating the composition of vapor in mass percent

  1. Feb 7, 2009 #1
    1. The problem statement, all variables and given/known data

    Here's the question form the text. (I have the answer, but don't know how to do part c.
    "A solution contains 50.0g of heptane (C7H16) and 50.0g of octane (C8H18) at 25 C. the vapor pressures of pure heptane and pure octane at 25 C are 45.8 torr and 10.9 torr, respectively. assuming ideal behavior, calculate each of the following:
    a) vapor pressure of each of the solution components in the mixture
    b) the total pressure above the solution
    c)the composition of the vapor in mass percent.

    the answer to a is: Pressure of C7H16: 24.4 torr
    Pressure of C8H18: 5.09 torr

    b) total pressure is: 29.49 torr.

    I have no idea how to go about part c. Any help is appreciated.

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 7, 2009 #2
    Remember that the partial pressure of a gas is directly proportional to the concentration of the gas. You know the fraction of the total pressure that the partial pressure of each substance comprises, which is equal to the mole fraction of each gas. You can then convert this into the mass percent of each gas.
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