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 175 mL of a buffer that is 0.180 M in both benzoic acid (CsHsCOOH) and its conjugate base (C6H5COo). Calculate the maximum volume of 0.340 M HCl that can be added to the buffer before its buffering capacity is lost. Number 42.06 mL

Explanation / Answer

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

pH = pKa + log [salt / acid]

    = 4.20 + log [0.180 / 0.180]

pH = 4.20

calculate concentration when pH = 3.20

pH = pKa + log [C6H5COO- /C6H5COOH]

3.20 = 4.20 + log [C6H5COO- /C6H5COOH]

[C6H5COO- /C6H5COOH] = 0.1

moles of acid and base present = 175 x 0.180 = 31.5

31.5 - x / 31.5 + x = 0.1

31.5 - x = 3.15 + 0.1 x

x = 25.77

moles of acid added = 25.77

Molarity = 0.340 M

volume = 25.77 / 0.340 = 75.80

volume of HCl = 75.8 mL

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