Question

When 1.0 ml of dil. HCl is added to 100 ml of a buffer solution of pH 4.0, the pH of the solution:
A.Becomes 7
B.Does not change
C.Small change
D.Becomes 10

Answer 1

Hint: A buffer solution is an aqueous solution that consists of a mixture of a weak acid and its conjugate base or a mixture of a weak base and conjugate acid. The pH of the buffer is kept constant even when it is used in different chemical applications.

Complete step by step answer:
We already know that buffers can be either acidic or alkali.
As mentioned above, it can be either a weak acid-conjugate base or weak base-conjugate acid. The conjugate base is the salt of a weak acid with a strong base (example: ${{C}}{{{H}}_{{3}}}{{COONa}}$)
If weak acid is present, then the pH of the buffer will be less than 7.
Now, the pH of the buffer is given as 4 and which is taken in 100ml. On adding a very small amount (given 1ml) of dil HCl to the buffer of 100ml, there will not be any change in the pH.
For example, if we take an acidic buffer, acetic acid, and sodium acetate, we know that acetic is a weak acid(as acetic acid does not completely dissociate into its ions in solution) and sodium acetate is its conjugate base.
On adding a small amount of dil. HCl, the reaction will be ${{C}}{{{H}}_{{3}}}{{COONa + HCl}} \to {{ C}}{{{H}}_{{3}}}{{COOH + NaCl}}$ , which forms a weak acid
Thus, the pH will remain the same.
${{pH = p}}{{{K}}_{{a}}}{{ + log}}\dfrac{{{{[}}{{{A}}^{{ - }}}{{]}}}}{{{{[HA]}}}}$ where ${{{A}}^{{ - }}}$ is the weak acid and HA is the conjugate base
Thus, on adding a very small amount of dil acid to an acidic buffer, the pH does not change. Here, only a volume of 1 ml is added to a buffer of 100ml. So there is no change in pH.
The correct option is (B).

Note:
On adding a strong acid, there will be a small change in the pH. Similarly, if we add a weak base to an alkali buffer, there will not be any change in the pH of the buffer. Thus we can conclude that the pH of a buffer solution does not change on dilution. Buffers are used in everyday life. Citric acid and sodium hydroxide are used for shampoo. The bicarbonate buffering system regulates the pH of the blood.