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, 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 5.07. 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

PH = Pka + log[acetate]/[acetic acid]

PKa = 4.74

PH = 5.07

5.07 = 4.74 + log[Acetate]/10

log[Acetate]/10   = 5.07-4.74

log[Acetate]/10    = 0.33

[Acetate]/10         = 100.33

[Acetate]/10         = 2.138

[Acetate]             = 2.138*10

[Acetate]            = 21.38millimoles

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