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Hess' Law - I need HELP!

  1. Dec 1, 2004 #1
    Compute the heat for formation of one mole of SO3 gas from the following data:

    Sorry I don't know how to do subscripts....

    S8(s) + 8O2(g) --> 8SO2(g) + 2378.4 kJ
    SO2(g) + 1/2O2(g) --> SO3(g) + 99.0 kJ

    I don't have a clue where to start....
  2. jcsd
  3. Dec 2, 2004 #2
    Since you are finding out the heat for formation of one mole of SO3 gas, it is necessary for you to find out the reaction equation for the foramation of SO3 gas using the given data first. By addition/subtraction/multiplication/division the given data, you can find out the equation, and therefore find out the heat for formation.
  4. Dec 2, 2004 #3
    So is this right?

    I'm going to multiply the second equation by 8....

    S8(s) + 8O2(g) --> 8SO2(g) + 2378.4 kJ
    8SO2(g) + 4O2(g) --> 8SO3(g) + 792 kJ

    Balanced equation (well adding the both of them together):

    S8(s) + 12O2(g) --> 8SO3(g)

    -2378.4 kJ + (-792 kJ) = -3170.4 kJ

    -3170.4 kJ/8 mol x 1 mol = -396. 3 kJ (the mol's will cancel)
  5. Dec 3, 2004 #4
    Um, Yes. I think it is right!
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