# General Chemistry Problem help

Discussion in 'DAT Discussions' started by Nasem, Apr 13, 2007.

1. ### Nasem 2+ Year Member

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An ideal solution of toluene (vapor pressure 21 torr) and cyclohexane (vapor pressure 67 torr) in which the mole fraction of cyclohexane is 0.74 is boiled and the vapor condensed to a liquid. What is the mole fraction of cyclohexane in the condensed liquid?

I can't figure this darn thing out... I always thought that the mole fraction of a solution (regarding phase) is the same no matter what... So I thought the answer would be 0.74 and its not....

Can anyone help plz

2. ### DentalKitty 5+ Year Member

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The only thing I can think of is that they heated it at a temp high enough to boil the cyclohexane, but not high enough to boil the toluene (about 20 deg C difference in bp's). This would make the vapor have a higher concentration of cyclohexane than in the original solution. As for how to get a mathematical answer from that, I am not sure. Unless the answer is 1.0 cyclohexane (which I'm sure it's not). If anyone knows, please help us out

3. ### poc91nc Membership Revoked Removed 7+ Year Member

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We are given the partial pressures for both compounds. So, the total pressure above the solution must be 21 + 67. Given the information can't we just use the equation for partial pressure?

88torr(mol fraction of toulene) = 21 torr = partial pressure of toluene

21/88 = mole fraction of toluene in the vapor above the solution. That's how I would do it...

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whoops...you need partial pressure for cyclohexane...just do the opposite then.

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total pressure = 88
partial pressure of cyclohexane 67

88(mole fraction of cyclohexane in vapor) = 67

67/88

6. ### poc91nc Membership Revoked Removed 7+ Year Member

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Just read the question again. Actually, we have to calculate the partial pressures first...thought they were given.
67(0.74) = 49.5
21(1 - 0.74) = 20.3
total pressure = 69.8
69.8 (mole fraction of cyclohexane) = 49.5

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Damn...the second partial pressure is 5.46....put the wrong number in the calculator. So....total pressure is 52.8

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uhghh...nevermind...it's 54.96. I haven't used a calculator in so long (on purpose) I think I forgot how to use it. But yeah...there is a similar problem in one of the Achiever tests and that is how you solve it (but I think they gave you the partial pressures). Calculating them seperately would be too time consuming for a practice test...and the numbers aren't very friendly.

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