1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Raoult's law - Vapor pressure

  1. Jun 6, 2010 #1

    Wek

    User Avatar

    The density of toluene is 0.862g/ml at 25C and it's vapor pressure s 28.44 mmHg. At 25C, the vapor pressure of toluene above a solution of C10H8 in 500ml toluene is 27.92 mmHg. How many grams of C10H8 are present in the solution?

    I first used Raoult's equation to find X. Then I used the density and given volume of toluene to find the grams and then the moles of it. Then using the moles fraction equation I found the moles of C10H8 and using the FW of it I found the grams of it.

    I did the work and come up with 12.8g, which is wrong. The book says it's 8.03g.

    Can anyone tell me what I might have done wrong? Thanks
     
  2. jcsd
  3. Jun 7, 2010 #2

    Borek

    User Avatar

    Staff: Mentor

    Sounds like your approach is a correct one, show your numbers.

    I didn't get neither 12.8g nor 8.03g :confused: Perhaps it is too early.
     
  4. Jun 7, 2010 #3

    Wek

    User Avatar

    This is what I did:

    -To find mole fraction of toluene I divided the vapor pressure of toluene solution by toluene's vapor pressure: 27.92/28.44 = 0.9817
    -To get the moles of toluene: density x toluene volume then divided by toluene FW: 431g/92.0g = 4.68mol
    -Using the moles fraction equation I found the moles of C10H18: C10H18 mol = (4.68/0.9817) - 4.68 = 0.10mol
    -To get the mass of C10H18 I multiplied the moles of C10H18 by its FW: 0.10mol x 128g = 12.8g
     
  5. Jun 8, 2010 #4

    Borek

    User Avatar

    Staff: Mentor

    1. Are you rounding down intermediate results? If so - don't.

    2. C10H8 or C10H18?

    Check your math.

    You are not getting 8.03g, but (after taking into account math errors) you are very close to my result. Either we are both wrong, or the book is wrong. I tend to think we are right.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook