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

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4. hmm I think you are something wrong

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

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