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 0.140 M in both acetic acid (CH3COOH) and its conjugate base (CH3COO–). Calculate the maximum volume of 0.330 M HCl that can be added to the buffer before its buffering capacity is lost.

Answer 1

Buffer solution of acetic acid and acetate ions will resist a change in its pH during addition of HCl by reacting acetate ions with HCl and forming weak electrolyte acetic acid, which dissociates to small extent and does not cause significant change in pH. This is called buffer capacity. The buffer capacity will be completely lost when all the acetate ions are consumed by the added HCl acid.

Therefore, buffer capacity can be calculated by calculating moles of acetate ions present which can completely react with HCl.

CH₃COO^- + HCl -----> CH₃COOH + Cl^-

Number of moles of acetate ions = Volume of buffer(L) x concentration of acetate ions (mol/L)

Number of moles of acetate ions = (0.175 L) x (0.140 mol/L)

Number of moles of acetate ions = 0.0245 mol

Thus, volume of 0.330M HCl required to react completely with 0.0245 mol of acetate ions can be calculated as follows:

Volume of HCl = 0.0245 mol / (0.330 mol/L)

Volume of HCl = 0.0742 L

Volume of HCl = 74.2 mL

Therefore, volume of 0.330M HCl required for buffer to loose its capacity is 74.2mL