Homework Help: Chemistry

Posted by Trixie on Tuesday, March 16, 2010 at 11:52pm.

A buffer solution of volume 100.0 mL is 0.150 M Na2HPO4(aq) and 0.100 M KH2PO4(aq). Refer to table 1.
(a) What are the pH and the pH change resulting from the addition of 80.0 mL of 0.0500 M NaOH(aq) to the buffer solution?
pH

pH change (include negative sign if appropriate)

(b) What are the pH and the pH change resulting from the addition of 10.0 mL of 1.0 M HNO3(aq) to the initial buffer solution?
pH

pH change (include negative sign if appropriate)

So I found the pH of the initial buffer solution to be 7.38 (using pka2=7.21). I am not sure how to set up this problem to add the NaOH to the buffer solution

Chemistry - Trixie, Wednesday, March 17, 2010 at 12:03am
for part a I tried using the HH equation, found moles of NaOH and used that for [base] and moles Na2HPO4 as [acid]...i used pKa 7.21...that was incorrect. For part b I used a similar method...plugging in HNO3 as the [acid] and Na2HPO4 moles as [base]

the other pka values for H3PO4 are:
Ka1 = 7.6E-3 ...... pKa1= 2.12
Ka2 = 6.2E-8 ...... pKa2= 7.21
Ka3 = 2.1E-13 .....pKa3= 12.68
Chemistry - Trixie, Wednesday, March 17, 2010 at 12:33am
I figured out the correct new pH for each respective part, but apparently the initial pH i found for the buffer solution is incorrect. I just need help in figuring that so I can calculate the change.

I initially thought the buffer pH was 7.38. Thanks!
Chemistry - Trixie, Wednesday, March 17, 2010 at 12:35am
NVM i was using the pKA instead of pH....i had it right afterall...guess it's jsut time for bed....
Chemistry - Anonymous, Tuesday, March 22, 2011 at 1:03am
what's the answer then?

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