[arcanum]

I've been trying at this problem for awhile and I don't know if its me or the book. Most likely me, since I've tried a couple of different problems and I couldn't get the answer they had.

Anyways the total question is here:

Calculate the pH of a solution that is made up to contain the following analytical concentrations: 0.0500 M in H₃PO₄ and 0.0200 M in NaH₂PO₄.

What I've done is below the line:

--------------------------------------
H₃PO₄ + H₂O [ H₂PO₄^- + H₃O⁺

Ka1 = 7.11E(-3)

Where 7.11E(-3) = H₃O⁺H₂PO₄^- / H₃PO₄

= 7.11E(-3) = (0.0200 + H₃O⁺) H₃O⁺ / (O.O500 - H₃O⁺

0 = [H₃O⁺] ² + (7.11E(-3) + 0.0200)H₃O⁺ - (7.11E(-3)(0.0500)

-------------------------------------------

Now where I get stuck is doing the darn quadratic equation on this...What I've done on this is used the standard equation:

-b (+-) (b² - 4ac)^1/2 / 2a

and I set a = 1  b = (7.11E(-3) + 0.0200) c = (7.11E(-3)(0.0500)

And the answer is suppose to be H₃O⁺ = 9.67E(-3)

Which every time I get something completely different. And like I said I've tried on a few different problems to use this equation but it doesn't seem to work.

I always tried using Chembuddy and the only equation I could find that would seem similar was

- Ka + (Ka² + 4 Ka Ca)^1/2 / 2

But plugged in that in and it didn't even give a close answer as well.

Anyone can see what I'm doing wrong or know how to do this sort of problem properly? I would be very thankful.

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch

[arcanum]

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch

Umm not quite what I was driving at in my problem. I showed my friend and he made the same mistake at first then realized that both of us were doing it correct, except that we didn't have our "C" value negative, since it was - (0.0500)(7.11E(-3) )

Which that was all that was incorrect with the calculations, otherwise everything else made sense. Thanks for the page, I already use Chembuddy for some stuff, but this was just purely a calculation mistake. Thanks for the response :+)