The ionisation constant of formic acid HCO2H is 1.8× 10^-4.what is per...
Calculation of Percentage Ionisation of Formic Acid
Giving the Given Data:
- Ionisation constant of formic acid (HCO2H) = 1.8 × 10^-4
- Concentration of formic acid (HCO2H) = 0.001 M
Formula:
Ionisation constant (Ka) of an acid is the ratio of the concentration of the dissociated ions to the concentration of the undissociated acid. Mathematically,
Ka = [H+][A-]/[HA]
where [H+] is the concentration of hydrogen ions, [A-] is the concentration of the conjugate base and [HA] is the concentration of the acid.
The percentage ionisation of an acid can be calculated by the following formula:
Percentage Ionisation = (Concentration of dissociated ions / Concentration of acid) × 100
Solution:
Firstly, we need to calculate the concentration of hydrogen ions (H+) and the concentration of the conjugate base (A-).
Since formic acid (HCO2H) is a weak acid, it dissociates partially in water, giving H+ and HCO2- ions.
Let the concentration of H+ be x.
Then, the concentration of HCO2- will also be x.
The concentration of undissociated formic acid (HA) will be (0.001 - x).
Therefore, the ionisation constant (Ka) can be calculated as:
Ka = [H+][A-]/[HA]
1.8 × 10^-4 = x × x / (0.001 - x)
On solving the above equation, we get x = 0.0085 M
Now, we can calculate the percentage ionisation of formic acid as:
Percentage Ionisation = (Concentration of dissociated ions / Concentration of acid) × 100
Percentage Ionisation = (0.0085 / 0.001) × 100 = 0.85%
Conclusion:
The percentage ionisation of a 0.001M solution of formic acid (HCO2H) is 0.85%.