Calculating pH from deltaG° and Ion Concentrations

[RNix25]

I've been working on this one for a few days, and I just can't seem to get the right answer.

Consider the reaction I2(s) --> I- (aq) + IO3- (aq)
Balanced= 3I2 + 6OH- --> 5I- + IO3- + 3H2O

deltaG°= -153.8 kJ/mol

The question: What pH is required for the reaction to be at equilibrium at 298 K when [I-]= 0.5 M and [IO3-]= 0.5 M

I know the answer is 9.21, but I just can't figure out how to get there. I started by finding Keq, which is 9.113 * 10^26, but I'm stumped from there. I would be extremely appreciative for a walkthrough to the final answer.

 

[uby]

Sounds like this should go in the homework help section.

Your K value looks correct. Write out K in terms of the concentrations of species raised to the power of their stoichiometric coefficients and solve for [OH-]. pH = 14 + log[OH-], I get 9.206 using your values.

 

[RNix25]

I slept on it and when I woke up, I looked over it and figured out where I was going wrong. Had one of my concentrations wrong, so I kept getting the wrong value when I worked it. I already had the answer, I just couldn't figure out why my math wasn't producing it. Anyway, your method was exactly right, so thanks for the help.

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