by Bipolarity
 Admin P: 23,533 How many decimal digits in your "7"?
P: 783
 Quote by Borek How many decimal digits in your "7"?
When my calculator evaluated a pH of 7, I think it was about 9-10 decimal places, so something like 7.000000000

I am pretty sure the result was exactly 7. Not completely sure, but I think if I ran it on Maple with 50 s.f. it would come out to be 7.000000000000000000000000000000000000000000000000000000000000000000000 0

I will recheck and let you know. Have you done the problem?

Thanks by the way.

BiP

 Admin P: 23,533 Titration Paradox! No, I have not done the problem, but I know what to expect. Your answer (zillions of zeros) is wrong. Show how you got it.
P: 21
You probably did a numerical error in the total volume, but that's not important, still you get closer to 7 than you actually should. There is more fundamental fallacy behind the calculation. You rely too much on the simplified equation pH = -log c (or pOH = -log c in the basic case). This equation does not account for the ions from dissociation of water which become relevant in very dilute solutions.

Let me simplify your problem:
What is the pH of 10-7M HCl? Is it seven? No! Or pH of 10-8M HCl, that would be even basic! And pure water, being 0M HCl would have infinite pH!

Solution?
You have to do so called charge balance (I'll do it for the simple case of HCl solution):
[H+] = [OH-] + [Cl-],
then substitute from water ionic product, and assume HCl is fully dissociated:
[H+] = Kw/[H+] + c

So you have quadratic equation for [H+]. Solve it, discard physically irrelevant root, and take -log of the remaining. You will get correct pH even for very dilute solutions.

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