Buffer capacity is a measure of a buffer solution\\\'s resistance to changes in
ID: 1027431 • Letter: B
Question
Buffer capacity is a measure of a buffer solution's resistance to changes in pH as strong acid or base is added. An ideal buffer has equal concentrations of acid and conjugate base for a [base] to [acid] ratio of 1. A buffer has lost its capacity when the ratio of [base] to [acid] is less than 0.10 or greater than 10. Suppose that you have 165 mL of a buffer that is 0.460 M in both hydrofluoric acid (HF) and its conjugate base (F–). Calculate the maximum volume of 0.120 M HCl that can be added to the buffer before its buffering capacity is lost.
Explanation / Answer
pKa of HF = 3.20
pH = pKa + log [NaF / HF]
= 3.20 + log [0.460 / 0.460]
pH = 3.20
buffer capacity lost when pH of a buffer changes by 1 unit
2.20 = 3.20 + log [F- / HF]
[F- / HF] = 0.10
moles of acid and base present = 165 x 0.460 = 75.9
75.9 - x / 75.9 + x = 0.10
75.9 - x = 7.59 + 0.1 x
x = 62.1
moles of acid = 62.1
volume of HCl = 62.1 / 0.120 = 517.5 mL
volume of HCl = 517.5 mL
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