Buffer Capacity

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 175 mL of a buffer that is 0.480 M in both benzoic acid (C₆H₅COOH) and its conjugate base (C₆H₅COO^-). Calculate the maximum volume of 0.410 M HCl that can be added to the buffer before its buffering capacity is lost.

Answer 1

The buffering capacity will be completely lost as soon as there is no more conjugate base (C₆H₅COO^-). The actual calculation of "buffer capacity" is different from what is being asked here (see below). So, to answer the question as it is asked, we will have to determine the volume of 0.410 M HCl needed to react completely with the conjugate base.

moles conjugate base = 175 ml x 1 L/1000 ml x 0.480 mole/L = 0.084 moles C₆H₅COO^-

Reaction taking place is C₆H₅COO^- + HCl ==> C₆H₅COOH + Cl^-

moles HCl needed = 0.084 moles C₆H₅COO^- x 1 mol HCl/mol C₆H₅COO^- = 0.084 moles HCl needed

Volume of HCl needed: (x L)(0.410 mol/L) = 0.084 moles

x = 0.2049 L = 204.9 mls = 205 mls of HCl

NOTE: Buffer capacity is generally defined as the moles of an acid or base necessary to change the pH of a solution by 1 when added to 1 liter of buffer.