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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: RNix25 on April 11, 2012, 05:01:49 PM

Title: Calculating pH from deltaG° and Ion Concentrations
Post by: RNix25 on April 11, 2012, 05:01:49 PM

I've been working on this one for a few days, and I just cant 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 of to the final answer.

Title: Re: Calculating pH from deltaG° and Ion Concentrations
Post by: JustinCh3m on April 11, 2012, 05:21:39 PM

set up the equilibrium expression.  the only unknown will be the denominator, [OH-].  Solve for [OH-] and you should be able to figure it out from there.

PS - I'm assuming your value for Keq is correct.

Title: Re: Calculating pH from deltaG° and Ion Concentrations
Post by: RNix25 on April 12, 2012, 04:32:20 PM

Thanks for responding. I had my one of my concentrations off, so I kept getting the wrong pH. Still, the quick response is appreciated.