1. Initial Region Weak Acid / Strong Base
1. The initial region
The titration curve shows the pH change vs the amount of titrant added. In a weak acid/strong base titration, the initial region before any base is added, the pH can be calculated for the aqueous solution containing only the weak acid.
Weak acid dissociation
A weak acid does not dissociate completely and the amount of dissociation is determined by its dissociation constant, Ka:
- Where HA is the weak acid
- H+ is the hydrogen ion
- A- is the conjugate base
Use an ICE table for the pH calculation
pH is a measure of the concentration of hydrogen ions (H+) in a solution. So, to find the H+ concentration of a weak acid in solution, you will need to use the acid's dissociation constant, Ka, and an ICE table.
Example: A 50.0 mL of (0.200 M) weak acid titrated with (0.0500 M) strong base. Calculate the initial pH of the solution given Ka = 1.8 x 10⁻⁵.
- Set up the ICE table and plug in the initial values.
- Use the stoichiometric ratio in the equation and work out the change using +x and -x
- Use the Ka expression and equate that the Ka value given
- Assuming x is very small, (0.200-x) is approximated to 0.200
- Simplify equation: X² = (1.8 x 10⁻⁵ x 0.200)
- Solve for X
- X = 0.0019
- pH = -log[0.0019] = 2.72