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Help with partial gas law problem

  1. Apr 1, 2013 #1
    1. The problem statement, all variables and given/known data
    A 32.0 mL sample of hydrogen is collected over water at 20°C and 750.0 torr pressure. What is the volume of the dry gas at STP? (vapor pressure of water at 20°C = 17.5 torr)


    2. Relevant equations

    Ptot = Pdry + Pwet

    P1V1/T1 = P2V2/T2

    3. The attempt at a solution

    Pdry = Ptot - Pwet = 750 - 17.5 = 732.5 torr

    V1 = P2V2T1/P1T2 = (732.5 torr)(32 mL)(298K)/(760 torr)(293 K) = 31.3 mL

    The answer in the answer key gives 28.7 mL
     
  2. jcsd
  3. Apr 2, 2013 #2

    DrClaude

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    Staff: Mentor

    What is STP?
     
  4. Apr 2, 2013 #3

    SteamKing

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  5. Apr 2, 2013 #4

    DrClaude

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  6. Apr 2, 2013 #5
    STP is standard temperature and pressure -- which I used
    T = 25.0 °C ( 298 Kelvin) and
    P = 1 atm of pressure (760 torr)

    Did I use these in the correct variables? Perhaps that is where I am confused?
     
  7. Apr 2, 2013 #6

    DrClaude

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    That's SATP. STP is T = 0°C.
     
  8. Apr 2, 2013 #7
    Ah, what a silly mistake. It works out now!

    Thanks so much DrClaude! I appreciate your time.
     
  9. Apr 3, 2013 #8

    DrClaude

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    You would be surprised how commen this mistake is. This is probably what SteamKing was alluding to. the main problem is that most thermodynamic is for 298 K, but STP was defined as 273 K.

    You're welcome!
     
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