Strengths of Weak Acids and Bases, K_a, K_b, pK_a and pK_b HL Only

Full Notes

Weak Acids and Bases

Weak acids and bases partially ionize in aqueous solutions. The stronger the weak acid or base, the more they dissociate in solution.

Their strength is measured using equilibrium constants:

Acid and Base Dissociation Constants

K_a is the equilibrium constant for the ionization of a weak acid:

K_b is for a weak base:

The larger the K_a or K_b, the more the substance dissociates – and the stronger the acid or base.

pK_a and pK_b Values

To make comparisons easier (since K_a and K_b values can be very small), we use the logarithmic forms pK_a and pK_b:

You can also calculate K_a from pK_a using:

You can also calculate K_b from pK_b using:

So:

• A smaller pK_a means a stronger acid.
• A smaller pK_b means a stronger base.
• At 298 K: pK_a + pK_b = 14 (for conjugate acid–base pairs).

Comparing Strengths

When comparing acids or bases, always compare the same form of the constant:

• K_a vs K_a (or pK_a vs pK_a)
• K_b vs K_b (or pK_b vs pK_b)

Never compare K_a with pK_a or K_b with pK_b.

Matt’s exam tip

Never compare K_a values with K_b values . Because they describe different chemical processes, comparing K_a and K_b directly (e.g. saying one is “stronger” based on a higher numerical value) is misleading. Instead, use pK_a and pK_b for a clearer comparison (lower values = stronger acid/base), or convert one into the other using: K_a × K_b = K_w (at 298 K).

Summary

• Weak acids and bases partially ionize in water.
• K_a and K_b measure their extent of ionization.
• Higher K_a or K_b = stronger acid or base.
• pK_a and pK_b are logarithmic forms, lower values mean stronger acids or bases.
• At 298 K, pK_a + pK_b = 14 for conjugate pairs.
• Always compare values of the same type (K_a with K_a or pK_a with pK_a).