[yoyo]

Using the generic reaction HA(aq)<----> H+(aq) + A-(aq), what is the concentration of [A-] if the ΔG˚= -340 kJ/mol at 25˚ C, if [HA]=0.08 M?

Using this formula:
ΔG˚=-R*T*lnK
where K=([H+]*[A-])/[HA]

I was able to obtain the value of K, which came out to be 3.97x10^59.

So that would mean 3.97x10^59= ([H+]*[A-])/(0.08M)

Multiplying K by [HA] gives 3.17x10^58.

So that would mean [H+]*[A-]= 3.17x10^58

The thing is that I'm only looking for [A-]. Am I to assume that [H+]=[A-] and then just divide 3.17x10^58 by 2 to get [A-]?
That would equal 1.59x10^58.

Or am I doing this completely wrong?

[Borek]

I was able to obtain the value of K, which came out to be 3.97x10^59.

Doesn't look correct to me. Equation is OK, but the result seems to be off. I have never seen that high dissociation constant.

[yoyo]

I was able to obtain the value of K, which came out to be 3.97x10^59.

Doesn't look correct to me. Equation is OK, but the result seems to be off. I have never seen that high dissociation constant.

Yeah, exactly. The same thought occurred to me as well, so I had a friend run through it as well (without him seeing any of my work so as to not influence the answer) and he came up with the same number. Am I doing something wrong with the calculating or do you think this may be a mistake on my professor's part?

[yoyo]

Okay, just to make sure I'm doing the calculations correctly, here goes:

-340/(-0.008314*298)=137.23=lnK

e^137.23= 3.97x10^59=K

I think the most likely explanation is that my professor just made up a bunch of numbers for the sake of the problem and hadn't realized that he'd taken it to astronomical proportions.

[Borek]

Value seems off, but I can't see anything wrong with your calculation of K.

Calculated concentration of A^- seems absurd as well. My guess is that [HA]=0.08M is to be read "analytical concentration of the acid" - that is, [HA] + [A^-] = 0.08. But if you have quoted question exactly as it was given to you it is just lousy worded and your answers are - technically - correct. Even if they don't make any sense.

In teh case it is not the correct answer, you may argue that you were not asked to guess what the professor meant, but to solve the question as given