What is the pH of a solution with a weak acid and its conjugate base?
What is the pH of a solution that is 0.100 M in the weak acid HA (Ka = 4.28×10⁻⁹) and 0.400 M in A-?
Final answer: The pH of the solution is approximately 4.68.
Calculating pH of a Solution with a Weak Acid and Conjugate Base
Initial concentration of HA (acid): 0.100 M
Initial concentration of A- (conjugate base): 0.400 M
Acid dissociation constant (Ka) for HA: 4.28×10⁻⁹
Solution:
To determine the pH of the solution, we can use the ICE table method in combination with the Ka expression for weak acids.
ICE Table Setup:
| HA | A- | |
|---|---|---|
| Initial (M) | 0.100 | 0.400 |
| Change (M) | -x | +x |
| Final (M) | 0.100 - x | x |
Ka Expression:
The equilibrium expression for the dissociation of the weak acid HA is:
Ka = [A-][H+]/[HA]
Solving for pH:
By substituting the values into the Ka expression and solving for x, we find the concentration of H+ ions to be approximately 2.1 × 10⁻⁵ M.
Therefore, the pH of the solution can be calculated using the formula pH = -log[H+].
After calculation, the pH of the solution is determined to be around 4.68.