pKa Determination | IB Chemistry

🎯 Challenge Setup

Difficulty

🧮 Find pKₐ and Kₐ

Step 1 — Click on curve

Find the equivalence point volume

cm³

Step 2

Half-equivalence volume

cm³

Step 3 — Read from curve

pH at half-equivalence = pKₐ

Step 4

Calculate Kₐ = 10⁻ᵖᴷᵃ

Unknown Weak Acid vs NaOH ● Generate a curve

💡 Click on the curve to read pH at any volume. Use this to identify key points.

Clicked Volume

—

Clicked pH

—

Steps Done

0 / 4

Determining pKₐ from a Titration Curve

At the half-equivalence point, exactly half the weak acid has been neutralized
This means [HA] = [A⁻], so the Henderson-Hasselbalch equation simplifies to pH = pKₐ
This is the most reliable method for experimentally determining Kₐ of an unknown weak acid
The half-equivalence point is always in the buffer region — where pH changes slowly

Step-by-Step Method

Step 1: Find the equivalence point — steepest part of the curve
Step 2: Half-equivalence volume = ½ × equivalence volume
Step 3: Read pH at half-equivalence volume from the curve → this is pKₐ
Step 4: Calculate Kₐ = 10⁻ᵖᴷᵃ
Verification: check pKₐ is in the buffer region (relatively flat zone)

IB HL Exam Tips

Graph reading: the equivalence point is NOT at pH 7 for weak acid/strong base — it's above 7
Common mistake: reporting pH at the equivalence point instead of the half-equivalence point
Kₐ format: express in scientific notation (e.g., 1.8 × 10⁻⁵) with correct sig figs
Reasonableness check: weak acids have pKₐ between 2 and 12; Kₐ < 1