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.

Suppose that you have 115 mL of a buffer that is 0.340 M in both propanoic acid (C2H5COOH) and its conjugate base (C2H5COO–). Calculate the maximum volume of 0.450 M HCl that can be added to the buffer before its buffering capacity is lost.

First, find the number of moles of C2H5COOH and C2H5COO–.

HCl is a strong acid and will react with C2H5COO– to completion. Set up an ICE table to show how the moles of C2H5COOH and C2H5COO– change when x mol of strong acid is added.

This will cause the [base] to [acid] ratio to decrease. Once that ratio gets below 0.10, the buffer capacity is lost. So set [C2H5COO–]final/[C2H5COOH]final equal to 0.10 and solve for x, where x is the moles of HCl added.

Explanation / Answer

mol propanoic acid= mol conjugated base= 0.34 x 0.115= 0.0391 mol

C2H5COOH <-------> C2H5COO-

0.0391+x 0.0391-x

0.1=(0.0391-x)/(0.0391+x) ----> x=0.032 mol of HCl ----> V= 0.032mol/0.45M= 0.071 L ---> 71 mL

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