Making Buffers ( AQA A Level Chemistry )

• When the ratio of salt to acid is 1:1, log₁₀ 1 becomes 0, so pH = pKa
• The pK_a of ethanoic acid is 4.75, so this buffer should have pH of 4.75
• You can make any buffer by adjusting the ratio of salt to acid to match the difference between the pKa and the desired pH
• For example, suppose you wanted to make a buffer of pH = 4.25,

pH = 4.25

4.25 = 4.75 -0.50

[salt]/[acid] = 0.50

• The ratio of the salt to acid has to be 0.50, so following the same procedure, you would need to dissolve half the mass of sodium ethanoate in the same volume of ethanoic acid solution, i.e. 1.025 g