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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

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