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 135 mL of a buffer that is 0.360 M in both propanoic acid (C2H5COOH) and its conjugate base (C2H5COO–). Calculate the maximum volume of 0.180 M HCl that can be added to the buffer before its buffering capacity is lost.

Answer 1

For the buffering capacity to be completely lost, one needs to use up all of the acid or all of the conjugate base. In this case, it would be using up all of the conjugate base, since an acid, HCl, is being added.
C2H5COO- + H+ --> C2H5COOH
moles C2H5COO- present

= 0.135 L x 0.360 mol/L

= 0.0486 moles C2H5COO- present
moles H+ from HCl to completely convert this to C2H5COOH = 0.0486 moles H+
volume 0.180 M HCl to get 0.0486 moles H

= (x L)(0.180 mol/L) = 0.0486moles
x = 0.270 L
x = 270 ml