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February 13, 2016
Posted by **Kelsey** on Thursday, March 15, 2012 at 12:18am.

M203(s)---> 2M(s) + 3/2 O2(g)

info given for Gf(kJ/mol):

M203= -10.60

M(s)=0

O2(g)= 0

1.) what is the standard change in Gibbs energy for rxn as written in forward direction? (kJ/mol)

2.) What is the equilibrium constant (K) of this rxn, as written in forward direction at 298K?

3.) What is the equilibrium pressure of O2(g) over M(s) at 298K? (atm)

For 1.) I got 10.6 kJ/mol and this is correct

For 2.) I got K= -4.28 and it marked me wrong. Now I got an answer of .01387. Is this correct???

For 3.) I got PO2= 0.0016 atm and it marked me wrong......I don't know how it is wrong.

Could you please check #2 and #3 and tell me what I did wrong and what are the answers thanks.

- Chemistry -
**DrBob222**, Thursday, March 15, 2012 at 12:50amI agree with your answer to #2.

For #3, if we do

0.01378 = pO2^{3/2}

and I evaluate that as 0.05748 atm but that has too many s.f. and I would round to 0.0575 atm. I tried 0.05748^1.5 and that = 0.01378. Check that carefully.

- Chemistry -
**Kelsey**, Thursday, March 15, 2012 at 12:56amYes it is correct! Thanks! :) I see my mistake!

- Chemistry -
**Bonjour**, Tuesday, June 11, 2013 at 6:38pmhello mon amie

- Chemistry -
**marie**, Monday, June 2, 2014 at 12:38amBonjour mon ami... Ca va? La chemie est un excellent science n'est-ce pas? Alors je vais partir maintenant, aie une bonne vie!

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**Sophia**, Saturday, April 4, 2015 at 1:10pm@DrBob222 , can you explain how exactly you evaluated to get that number because im confused howd you got there

- Chemistry -
**Lore**, Sunday, April 26, 2015 at 12:59amI see what asker did wrong too, because she left it incomplete at first.

Since Kelsey found #1, 10.6, I'll leave it at that.

Part 2: (Gibbs energy)= -RTln(K)

-> 10.6 kJ/mol= -(8.314 J/molxK)(298K)ln(K)

-> 10.6 x 10^3 (J/mol) = -(2477.6 J/mol)ln(K)

-> Divide "-RT" (which is -2477.6 J/mol) from both sides

-> -4.278 = ln(K)

-> e^(-4.278) = K (because e and ln cancels out)

-> 0.0139

From what I see...

Part 3: pO2 is (pO2)^(3/2) because of the 3/2 moles in M203(s)---> 2M(s) + 3/2 O2(g). So...

->(pO2)= 0.0139

-> [(pO2)^(3/2)]^(2/3)= (0.0139)^(2/3)

-> pO2= 0.0578

- Chemistry -
**Lore**, Sunday, April 26, 2015 at 1:09amSorry, for part three, it is not only because of 3/2 moles from O2(g). Part 3 is asking for "equilibrium pressure of O2(g) over M(s) at 298K? (atm)." Although it is 3/2 nevertheless, because M(s) also has 1 mole, it is due to O2(g)/M(s) so (3/2)/(1) =3/2. Please correct me if my thinking is wrong.

- Chemistry -
**Lore**, Sunday, April 26, 2015 at 1:14amOh, how embarrassing for the spam, but what I stated earlier, just above, was a work of negligence. Because I quickly looked at the problem, I mistook M203 for M(s). If only there was an editing button...I'll just leave my work up in the open, as I was trying to follow DrBob222's reasoning.