1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Vapour pressure problem (simple)

  1. Jun 9, 2009 #1
    The vapour pressure of pure chloroform at 70.0oC is 135.7 kPa. What mass of iodine (I2) should be dissolved in 1 L of chloroform, CHCl3 (density = 1.49 g/cm3) to lower the vapour pressure by 13.3 kPa?

    hey guys i really cant do this question, i think in involves raults law, P=XPA
    anyhelp would be good. thanks

    p.s. answer is 1.32 x 101 g
  2. jcsd
  3. Jun 10, 2009 #2


    User Avatar

    Staff: Mentor

    No idea if you are to take into account iodine pressure or not.

    Assuming it has to be ignored - at what x the pressure of chloroform vapour will be lowered by 13.3 kPa?
  4. Jun 10, 2009 #3
    thnks for the reply, umm 122.4kpa
  5. Jun 17, 2009 #4

    Here is the solution I got, its different to your answer though...


    so Psolution=135.7-13.3 kpa (the new pressure of the solution)

    now in 1L =1000cm3 there is 1490 grams = 12.482 moles

    so re-arrange the equation and you get=> x = (12.482 - 11.26)/(3.55x10-3)

    =344.23 grams

    if 133.32 is the correct answer as you suggest I dont know where went wrong.
  6. Jun 17, 2009 #5
    hey mate i checked with the teacher and ur right(stupid internet tests). just wondering if you could show me where you got a few of the those numbers from, ive tried but get lost. thanks
  7. Jun 20, 2009 #6

    so Xsolvent=molsolvent/(molsolute+molsolvent), we are looking to find molsolute (I2) so we can multiply by the RFM (253.8 g.mol-1) to get the mass needed.

    1L of chloroform has mass 1490g = 12.482mol



    re-arranged x=1.356 mol

    therefor mass I2 = 344g
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook