If you are satisfied that dG is ok, then set up the ICE chart.
.......H2O ==> H^+ + OH^-
I......1.0.....0......0
C......-p......p......p
E......1.0-p...p......p
Then Kp = pH^+ * pOH^-/pH2O
and substitute the E line into Kp expression and solve for p. Then use p to calculate mole fraction or use PV = nRT to calculate mols and/or grams.
I looked in my text to find dGo for H^+ and OH^- but no luck. But if your text has those values it would be interesting to calculate dGo products - dGo reactants = dG rxn and plug into dG = -RTlnK and see how close the two K values are.
I agree that Ptotal initially is 1.0 atm from the problem. After the ICE chart it is what you have. It will be Ptotal = 1-p + p + p = 1+p so
XH2O = 1-p/1+p
XH^+ = p/1+p
XOH^- = p/1+p
For :H2O(g) --> H(g) + OH(g); At 3000K and P(total)=1.00 atm, from your K value calculate system composition from 1.00 mol H2O(g) and no products initially.
The K I got (using Delta G superscript zero = [Delta H superscript zero]-[T* Delta S superscript zero)=]) was 9.5x10^-4 atm.
I know system composition is mole composition, which is proportional to mole fraction. I think Ptotal=P(H2O)+p(H)+P(OH) but where do I proceed from there?
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