Ka and Kb Explained for IB Chemistry

A dilute strong acid still has a very high H⁺ concentration relative to a concentrated weak acid. Ka and Kb always refer to degree of ionization, not the amount of acid or base added.

Relationship Between Ka and Kb

For any conjugate acid–base pair:

Ka × Kb = Kw

Where Kw = 1.00 × 10⁻¹⁴ at 25°C.

This means:

• A strong acid (large Ka) has a conjugate base with a very small Kb
• A strong base (large Kb) has a conjugate acid with a very small Ka

This reciprocal relationship is frequently tested in Paper 2.

Using Ka and Kb to Determine pH

Ka and Kb allow you to calculate:

• [H⁺] for weak acids
• [OH⁻] for weak bases
• pH or pOH
• Degree of dissociation
• Concentration of ions in buffers

For a weak acid HA, if x is small:

[H⁺] ≈ √(Ka × [HA])

For a weak base B:

[OH⁻] ≈ √(Kb × [B])

These approximations work because weak acids and weak bases ionize only slightly.

Ka and Kb in Buffer Solutions

Buffers rely on conjugate acid–base pairs.
Their pH depends on the ratio of acid to conjugate base.

The Henderson–Hasselbalch equation (HL) uses Ka:

pH = pKa + log([A⁻]/[HA])

Similarly, for basic buffers:

pOH = pKb + log([BH⁺]/[B])

This shows how Ka and Kb control buffer behavior.

pKa and pKb

To simplify calculations, Ka and Kb are often converted to logarithmic form:

pKa = −log(Ka)
pKb = −log(Kb)

Interpretation:

• Smaller pKa → stronger acid
• Smaller pKb → stronger base

These values are easier to compare than Ka and Kb directly.

Trends in Ka and Kb

Stronger acids:

• Higher Ka
• Lower pKa

Stronger bases:

• Higher Kb
• Lower pKb

These trends help identify whether species behave strongly or weakly in water.

Common IB Mistakes

• Mixing up strength with concentration
• Forgetting that Ka and Kb apply only to weak acids and bases
• Using the wrong equilibrium expression
• Neglecting units (Ka and Kb are typically dimensionless in IB)
• Forgetting the relationship Ka × Kb = Kw

Careful setup of expressions prevents calculation errors.

FAQs

Do strong acids have Ka values?

Not in practice. Their ionization is essentially 100%, so Ka would be extremely large and not useful.

Does temperature affect Ka and Kb?

Yes. Both are equilibrium constants and vary with temperature.

Why do weak acids need Ka instead of simple stoichiometry?

Because they only partially dissociate, so equilibrium—not full dissociation—must be considered.

Conclusion

Ka and Kb are equilibrium constants that measure the strength of weak acids and weak bases. Ka shows how much an acid donates protons, while Kb shows how effectively a base accepts protons. These constants determine pH, relate conjugate pairs through Kw, and form the basis for buffer calculations. Mastering Ka and Kb is essential for tackling acid–base questions in IB Chemistry.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.