Question

Some properties of buffers

Buffer system selected HC₂H₃O₂ - C₂H₃O_{2 HB is acetic acid.}

1. pH of buffer: 4.44 [H⁺ ] __________M pKa___________

2. pH of diluted buffer 4.39 [H⁺] ________M pKa__________

Comment on your observations in Parts 1 and 2.

Answer 1

1. pH of buffer = 4.44

pH = -log[H+]

4.44 = -log[H+]

[H+] = 3.6*10^-5 M

pKa of acetic acid = 4.75

2. pH of buffer = 4.39

pH = -log[H+]

4.39 = -log[H+]

[H+] = 4.1*10^-5 M

pKa of acetic acid = 4.75

From both the solutions, it can be observed that pKa is characteristic of acid. It does not change with dilution or any concentration. It is a constant for a specific acid. For acetic acid, it is 4.75.

pH of both the solutions are different but does not differ very much. Although the concentration has been changed, but pH change is negligible. This is a property of buffer solution. Addition of acid or base does not change pH of the solution much. Water is also a weak acid, but the addition of water did not change pH much.