The pH of a 0.100 M solution of a weak acid
HA is 4.94 . Calculate Ka for the acid.
To calculate Ka (acid dissociation constant) for a weak acid, you need to know the initial concentration of the acid and the pH of the solution.
1. Start by writing the balanced equation for the ionization of the weak acid. The general equation for a weak acid HA is:
HA ⇌ H+ + A-
2. Identify the stoichiometric ratio between the acid and its ions. In this case, one molecule of HA dissociates into one H+ ion and one A- ion.
3. Use the pH of the solution to calculate the concentration of H+ ions. The pH scale is defined as the negative logarithm of the hydrogen ion concentration, [H+]. Therefore, [H+] can be calculated using the following equation:
[H+] = 10^(-pH)
In this case, [H+] = 10^(-4.94) = 3.24 x 10^(-5) M.
4. Since the concentration of H+ ions equals the concentration of A- ions (due to the stoichiometric ratio), the concentration of A- ions is also 3.24 x 10^(-5) M.
5. Calculate the concentration of the unionized weak acid, HA, by subtracting the concentration of H+ ions from the initial concentration of the weak acid. In this case, the initial concentration of HA is 0.100 M, so:
[HA] = 0.100 M - 3.24 x 10^(-5) M = 9.68 x 10^(-2) M.
6. Calculate Ka using the equation:
Ka = ([H+][A-]) / [HA]
In this case, Ka = [(3.24 x 10^(-5) M) * (3.24 x 10^(-5) M)] / (9.68 x 10^(-2) M)
Simplifying the expression gives Ka = 1.09 x 10^(-7) M.
Therefore, the Ka value for the weak acid HA is 1.09 x 10^(-7) M.