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Buffer capacity is a measure of a buffer solution\'s resistance to changes in pH as strong acid or base is added. Suppose that you have 165 mL of a buffer that is 0.360 M in both benzoic acid (C6H5COOH) and its conjugate base (C6H5COO–). Calculate the maximum volume of 0.250 M HCl that can be added to the buffer before its buffering capacity is lost.

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Brody
5 answers

  1. 165 mL x 0.360M = about 59.4 millimols base and 59.4 mmols acid.

    ..........base + HCl ==> acid
    I........59.4.....0......59.4
    add...............x...............
    C........-x.....-x........+x
    E.......59.4-x....0......59.4+x

    pH = pKa + log (base)/(acid)
    Look up the pKa for benzoic acid.
    Subtract 1.0 from that (remember buffer capacity is acid/base that can be added and not make more than +/- 1 for change of pH) and substitute that for pH. Substitute 59.4-x and 59.4+x for base and acid and solve for x = millimols HCl that can be added to make that difference of pH = 1.
    Then M = mmols/mL.
    You know M HCl, mmols HCl, solve for mL HCl.

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    DrBob222

  2. How do you solve for x? I keep getting a negative answer and I'm not sure that's right...

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    kpedenko

  3. hmm i think you just did something wrong kpedenko ?

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    Alawrence

  5. hmm I think you are something wrong

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    kpedenko

  6. kpedenko, recall that it is an acid being added to a base therefore your ph/pka difference should be .1

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    master chemist

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