Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 14.23 Core Practical: Ka determination-Study Notes - New Syllabus

14.23 CORE PRACTICAL 11: Finding the \( \mathrm{K_a} \) Value of a Weak Acid

This practical determines the acid dissociation constant (\( \mathrm{K_a} \)) of a weak acid using a titration curve. The key idea is that at the half-equivalence point, \( \mathrm{pH = pK_a} \).

Aim

To determine the value of \( \mathrm{K_a} \) for a weak acid using titration data.

Apparatus

• Burette
• Pipette
• Conical flask
• pH meter (or probe)
• Magnetic stirrer (optional)

Chemicals

• Weak acid solution (e.g. \( \mathrm{CH_3COOH} \))
• Standard \( \mathrm{NaOH} \) solution (strong base)

Procedure

1. Pipette a known volume of weak acid into a conical flask.
2. Place pH probe in the solution.
3. Add \( \mathrm{NaOH} \) from burette in small increments.
4. Record pH after each addition.
5. Continue until well past equivalence point.
6. Plot pH vs volume of base added.

Processing Results

Step 1: Identify Equivalence Point

• Point of steepest vertical increase in pH.

Step 2: Find Half-Equivalence Point

• Half the volume at equivalence.

Step 3: Read pH at this point

• \( \mathrm{pH = pK_a} \)

Step 4: Calculate \( \mathrm{K_a} \)

\( \mathrm{K_a = 10^{-pK_a}} \)

Theory

At half-neutralisation:

• \( \mathrm{[HA] = [A^-]} \)

From:

\( \mathrm{K_a = \frac{[H^+][A^-]}{[HA]}} \)

Therefore:

\( \mathrm{K_a = [H^+]} \Rightarrow pH = pK_a \)

Key Features

• Uses titration curve data.
• Half-equivalence point is critical.
• No need for complex equilibrium calculations.

Sources of Error

• pH meter calibration errors.
• Inaccurate volume readings.
• Adding base in large increments near equivalence.

Improvements

Use smaller volume additions near equivalence.

• Use a calibrated pH probe.
• Repeat experiment for accuracy.