Effects of Dilution on a Weak Acid Solution

When a 0.1M solution of a weak acid is diluted to 0.01M at constant temperature, several changes occur. Let's analyze each effect in detail:

1. % Ionisation will decrease:
- The term % ionisation refers to the extent to which the weak acid dissociates into its ions in solution.
- The equation for the dissociation of a weak acid (HA) can be represented as: HA ⇌ H+ + A-
- Initially, the 0.1M solution of the weak acid has a higher concentration, which promotes greater dissociation.
- However, when the solution is diluted to 0.01M, the concentration of the weak acid decreases, leading to a decrease in the number of dissociated ions.
- Consequently, the % ionisation of the weak acid will decrease.

2. [H+] will decrease to 0.01M:
- The concentration of hydrogen ions ([H+]) in a solution is determined by the dissociation of the weak acid.
- As stated earlier, the weak acid dissociates into H+ and A- ions.
- Initially, in the 0.1M solution, the concentration of [H+] is higher due to the higher concentration of the weak acid.
- However, upon dilution to 0.01M, the concentration of [H+] will decrease accordingly.
- Thus, after dilution, the [H+] will be 0.01M.

3. Ka will increase:
- Ka is the acid dissociation constant, which represents the strength of an acid.
- It is calculated as the ratio of the concentration of dissociated ions ([H+][A-]) to the concentration of the undissociated weak acid ([HA]).
- Ka = [H+][A-] / [HA]
- When the weak acid solution is diluted, the concentration of the undissociated weak acid ([HA]) decreases.
- As a result, the value of Ka will increase because the numerator ([H+][A-]) remains constant while the denominator ([HA]) decreases.
- Therefore, Ka increases upon dilution.

4. pH will decrease by 2 units:
- The pH of a solution is a measure of its acidity or alkalinity.
- It is determined by the concentration of hydrogen ions ([H+]) in the solution.
- The pH scale is logarithmic, meaning that each unit represents a tenfold difference in [H+] concentration.
- Initially, in the 0.1M solution, the [H+] concentration is higher, leading to a lower pH value.
- However, upon dilution to 0.01M, the [H+] concentration decreases, resulting in an increase in pH.
- Since the pH scale is logarithmic, a decrease in pH by 2 units means that the final pH will be higher (more alkaline) after dilution.

In summary, when a 0.1M solution of a weak acid is diluted to 0.01M at constant temperature:
1. The % ionisation will decrease.
2. The concentration of [H+] will decrease to 0.01M.
3. The Ka value will increase.
4. The pH will decrease by 2 units.