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Homework Help: Thermodynamics Questions

  1. May 27, 2007 #1
    Hey guys I was just wondering if you could help me out as I really struggle with these thermo questions and can never figure them out, cheers for any help :)

    Question 1)Air consists of 20% oxygen and 80% nitrogen (by volume). Calculate the Gibbs energy of mixing in Joules if 8 x 3.9 L of pure nitrogen is mixed with 2 x 3.9 L of pure oxygen at 1 atm pressure.

    I was thinking of using the fromula G = n*R*T*ln(Xa*ln(Xa) + Xb*ln(Xb)) but I can't see how the volumes come into it? I have no chemistry background at all and I think this is why I struggle :frown:

    Question 2)What is the minimum amount of energy in Joules which would be required to separate 2 x 1.6 L of pure oxygen from 10 x 1.6 L of air at 1 atm pressure?

    I think this will be the same as the first one but in reverse..

    Question 3)Calculate the number of grams of oxygen dissolved in 7 L of water at 25°C when it is in equilibrium with the air above it at 1 atm. (Assume Henry’s law applies and use K=4.34 x 104 atm. Note that the partial pressure of oxygen in air is 0.2 atm and 1L of water has a mass 1kg).

    Question 4)Liquid Acetone and Chloroform mixtures satisfy Raoults law approximately. If the vapour pressure at 35°C of pure acetone is 347 Torr and of pure Chloroform is 293 Torr, calculate the vapour pressure in Torr of a mixture with 28 % mole fraction of Chloroform.

    Cheers guys, I need all the help I can get :smile:
  2. jcsd
  3. May 27, 2007 #2


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    1) What's the relation between the volume fraction and mole fraction? Hint: Use the ideal gas law

    What have you tried for 3) and 4) ?
  4. May 27, 2007 #3
    No I haven't tried 3 and 4 but was thinking of using the formula pa = xa.pa*?
  5. May 28, 2007 #4


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    Have you read the definition of Henry's Law and do you understand what the terms mean? If you do that, you'll easily be able to figure out what to do.
  6. May 30, 2007 #5
    Hi, I still can't get the right answer so do you mind explaining the answers to me step by step as I need to know this and it would be good for revision. Cheers :)
  7. May 31, 2007 #6
    Ok for number 1 I have xa = .2 and xb = .8

    Therfore if I plug into eqaution what do I use for n? is it 1 adn what do i use for T, do i just use the room temp which would be around 300K?
  8. May 31, 2007 #7
    I used n = 1 and R = 8.314 and T = 300 and got -1248.1, does that sound correct?
  9. May 31, 2007 #8
    And for question 2 I just did the same but put xa = .167 and xb = .8333 and got that G = -1123.79???
  10. May 31, 2007 #9
    Question 3 I had 7000/18 = 388.89*.2/(4.34*10^4) = .001792? Is this right, I have tried to input the answers to all these and I still get it wrong :( Please I need help urgently!
  11. May 31, 2007 #10


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    Yeah, cause the mol frac is the same as vol frac for an ideal gas. (Use the ideal gas law to show this).

    No, you don't use 1 mol for n.
    AT STP, 22.4 L corresponds to 1 mol. So, 8 x 3.9 L corresponds to how much? Similarly, how many moles of Oxygen? So, what's the total no of moles?

    And you're right in using T as room temp.
  12. May 31, 2007 #11
    So I use n = 31.2/22.4 +7.8/22.4 = 1.74mol, but where did you get 22.4 from?
    Last edited: May 31, 2007
  13. May 31, 2007 #12
    I got number 4 right, Ptotal = Pb* + (Pa* - Pb*)xa but am still getting the rest wrong :S
  14. May 31, 2007 #13


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    Because, for 1 mol of gas, at 300 K, 1atm pressure, what do you get if you calculate V from the ideal gas law?

    For question 3, can you post hery's law here and explain what each term in that means? That'll help you get the answer.
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