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 0.160 M in both propanoic acid (C₂H₅COOH) and its conjugate base (C₂H₅COO^-). Calculate the maximum volume of 0.470 M HCl that can be added to the buffer before its buffering capacity is lost.

Answer 1

The pKa of propanoic acid is 4.87.

115 mL of a buffer that is 0.160 M in both propanoic acid (C2H5COOH) and its conjugate base

Number of moles of propanoic acid

Number of moles of conjugate base (propanoate ion) .

Now x moles of HCl are added so that pH changes from 4.87 to 3.87

x moles of HCl will react with x moles of propanoate ion to form x moles of propanoic acid.

Number of moles of propanoic acid

Number of moles of conjugate base (propanoate ion) .

Thus, 0.015055 moles of 0.470 M HCl are added.

Volume of 0.470 M HCl added

Convert the unit from L to mL.

Hence, 32.0 mL of 0.470 M HCl are to be added.

Note:

Buffer capacity is the number of moles of acid or base required by one litre of a buffer solution for changing its pH by one unit.