Question

Using Ka values given in your textbook, design a buffered solution whose pH is 4.50.

a) what acid and salt will you use to make this buffer?

b) if you start with 0.20M solution of the weak acid, what inital concentration of the salt is needed?

c) how many grams of th salt would be needed to make 1.000 L of the buffer solution?

Answer 1

a) Let us look at the acetate buffer formed using acetic acid and sodium acetate.

If we take a 0.6 M acetic acid CH3COOH, and 0.5 M sodium acetate, CH₃COONa, which is its conjugate base. The reason we are taking similar concentration is that the best buffers have similar concentration of acid and acid salt as they give better buffer capacity.

CH₃COOH CH₃COO^- + H⁺

The Ka for acetic acid is 1.76 x 10^-5.

Ka = [CH₃COO^-][H⁺] / [CH₃COOH]

Since acetic acid is a weak acid it will barely dissociate.

1.76 x 10^-5 = 0.5 x [H⁺]/ 0.6

[H⁺] = 2.11 x 10^-5

pH = -log[H+]

pH = -log(2.11 x 10^-5) = 4.67

If we need this buffer to have pH of 4.50 we can calculate it backward

4.50 = -log[H⁺] i.e. [H⁺] = 3.162 x 10^-5

putting this value in the Ka expression, assuming concentration of sodium acetate to be 0.5 M

1.76 x 10^-5 = 0.5 x 3.162 x 10^-5/ X

so x = 0.89 M acetic acid will be required.

b) If you take 0.2M acetic acid to get a pH 4.5 buffer we will need sodium acetate.

since pH -s 4.50

4.50 = -log[H⁺] i.e. [H⁺] = 3.162 x 10^-5

1.76 x 10^-5 = X x 3.162 x 10^-5/ 0.20

inital salt concentration will be 0.11 M

c) To make 1 L of the buffer solution we will need 0.11 x 82 g/mole of sodium acetate which is 9.02 g of sodium acetate.