Buffer solutions can be produced by mixing a weak acid with its conjugate base o

Question

Buffer solutions can be produced by mixing a weak acid with its conjugate base or by mixing a weak base with its conjugate acid. The Henderson-Hasselbalch equation, pH = pK_a + log [base]/[acid] allows you to calculate the pH of a buffer. Note that molarity, moles, and millimoles are all proportional, so you can substitute the number moles, or millimoles, for the concentration terms in this formula. You need to produce a buffer solution that has a pH of 5.13. You already have a solution that contains 10. mmol (millimoles) of acetic acid. How many millimoles of acetate (the conjugate base of acetic acid) will you need to add to this solution? The pK_a of acetic acid is 4.74. Express your answer numerically in millimoles.

Explanation / Answer

5.13 = 4.74 + log x / 10
=>5.13-4.74=log x - log 10
=>0.39=logx - 1

=>1.39=log x

=>x=antilog(1.39)

=>x=24.55 mmol of acetate

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