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 105 mL of a buffer that is 0.180 M in both propanoic acid (C2H5COOH) and its conjugate base (C2H5COO–). Calculate the maximum volume of 0.240 M HCl that can be added to the buffer before its buffering capacity is lost.

Answer 1

Buffer capacity

change in pH unit by 1.

let x amount of HCl is added

then by using Hendersen-Hasselbalck equation,

In the given buffer of C2H5COOH/C2H5COO-

[C2H5COOH] = [C2H5COO-] = 0.180 M

pH = pKa + log(base/acid)

pKa = 4.87

initial pH,

pH = 4.87

change in pH by 1 unit, pH = 3.87 by addition of base,

3.87 = 4.87 + log[(0.180 M x 0.105 L - x)/(0.180 M x 0.105 L + x)]

0.0189 + x

0.00189 + 0.1x = 0.0189 - x

x = 0.01701/1.1 = 0.0155 mol

so, moles of HCl to be added before buffer capacity is lost = 0.0155 mol

Volume of HCl to be added = 0.0155 mol x 1000/0.240 M = 64.58 ml