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Calculate the work in Joules by the Acetylene gas

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  1. Jan 27, 2015 #1
    1. The problem statement, all variables and given/known data
    A quantity of 4.50 g CaC2 is reacted with an excess of water at 298K and atmospheric pressure.
    CaC2 (s) + 2 H2O (l) --> Ca(OH)2 (aq) + C2H2 (g)
    Calculate the work done in Joules by the Acetylene gas against the atmospheric pressure.



    2. Relevant equations
    After asking my PChem professor, he said the I should use the work formula for an isothermic gas expansion, w= -nRT ln(V2/V1)
    also for an isothermic expansion, q=-w



    3. The attempt at a solution
    P= 1 atm, n= 0.0702 mol R= 8.314 J/mol*K
    I easily found the mole of the gas (0.0702), but my biggest problem is what are the volumes that are supposed to be used here? Using the moles of an ideal gas (22.4 L/mol), I found that the volume of the acetylene gas is 1.572 L, but I don't have a second volume to complete the equation. I feel like I'm missing something important, but I can't figure it out.
     
  2. jcsd
  3. Jan 27, 2015 #2

    Bystander

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    Can you recall another isothermal work process?
     
  4. Jan 27, 2015 #3
    OH!
    w= -nRT ln (P2/P1)

    So the P1 is atmospheric pressure and for the P2 I use PV=nRT
    Then I have both pressures and it's just plug and chug from there.
     
  5. Jan 27, 2015 #4

    Bystander

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    No, it's all at atmospheric.
     
  6. Jan 27, 2015 #5
    Did your professor really say that you should use the work formula for an isothermic gas expansion, w= -nRT ln(V2/V1)
    also for an isothermic expansion, q=-w?

    Neither of these suggestions is correct.

    Bystander is totally correct. Also, you weren't asked for the amount of heat transferred. But, if you did have to find it, you would be looking for the "heat of reaction," for the reaction under consideration.

    Chet
     
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