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A buffer contains 1.0mol CH3CO2H and 1.0mol CH3CO2^- diluted with water to 1.0L. How many moles of NaOH are required to increase the pH of the buffer to 5.10? (pKa oof Ch3CO2H=4.74)

  • Chem -

    You want to use the Henderson-Hasselbalch equation,
    pH = pKa + log (base/acid)
    First, plug in 5.10 for pH and 4.74 for pKa, and solve the H-H equation for B/A ratio.

    Then look at the equation if you add OH^- to the buffer solution. It will neutralize acid to produce more acetate at the expense of CH3COOH.
    CH3COOH + OH^- ==> CH3COO^- HOH

    Here is what I would do.
    Let x = moles OH^- to be added which is what the problem asks for.
    You are starting with 1 mole CH3COOH and 1 mole CH3COO^-
    At the end of x moles OH^-, the concns are
    1-x for CH3COOH (acid) and 1+x for CH3COO^- (base).
    Now plug those into the B/A you solved for above and solve for x.
    Then to make sure you are right, plug B and A back into the H-H equation and see if you end up with pH of 5.01. Post your work if you get stuck. It may sound complicated but it really is straight forward.

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