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March 27, 2017

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1.What is the pH of a solution containing 0.042 M NaH2PO4 and 0.058 M Na2HPO4 ? The pKa of sodium phosphate is 6.86. pH = 6.86 + log [0.058] / [0.042]; pH = 6.86 + 0.14; pH = 7.00
That is my answer to the above question. The following question is what I am having trouble with.
2.What is the pH of the sodium phosphate buffer described in problem #1 if 1 ml of 10M NaOH is added? What is the pH of 1 liter of water if 1ml of NaOH is added?

  • Chemistry - ,

    2a. Adding 1 mL of 10M NaOH to HOW MUCH of the buffer. Since the problem gives molarities, I will assume 1L.
    millimols HPO4^2- = 1000 mL x 0.058 = 58
    mmols H2PO4 = 1000 mL x 0.042 = 42
    mmols NaOH added = 1 mL x 10M = 10

    .......H2PO4^- + OH^- ==> HPO4^2- + H2O
    I......42........0........58.........0
    added...........10.................
    C......-10.....-10........+10.....+10
    E.......32......0.........68

    pH = 6.86 + log(68/32) = about 7.19

    2b.
    10M NaOH x (1/1001) = 0.00999M
    pOH = 2
    pH = 10
    Check my work for typos. The chemistry is ok.

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