Buffer capacity is a measure of a buffer solution\'s resistance to changes in pH

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 195 mL of a buffer that is 0.260 M in both hydrofluoric acid (HF) and its conjugate base (F Calculate the maximum volume of 0.160 M HCI that can be added to the buffer before its buffering capacity is lost. Number mL

Explanation / Answer

total moles of buffer = 195 x 0.260 / 1000 = 0.0507

pKa of HF = 3.20

pH = pKa + log [F- / HF]

     = 3.20 + log (0.260 / 0.260)

     = 3.20

pH = 3.20

when pH of a buffer capacity changes by 1 unit then the buffer capacity to be lost

when HCl is added , new pH will be less than 3.20

concentration when pH = 2.20

pH = pKa + log [F- / HF]

2.20 = 3.20 + log [F- / HF]

[F- / HF] = 0.1

[F- + HF] = 50.7

50.7 - x / 50.7 + x = 0.1

x = 41.48

mmoles of acid added = 41.48

molarity = moles / volume

0.160 = 41.48 x 10^-3 / volume

volume = 0.259 L

volume of acid = 259.3 mL

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