Strong vs Weak Acids: Ka, pKa and Conjugate Base Strength

A strong acid dissociates essentially completely in water. A weak acid only partly dissociates, and is described by Ka. pKa = -log Ka, so a lower pKa means a stronger acid.

Key relationships

• Ka x Kb = K_w = 10^-14 for a conjugate pair; pKa + pKb = 14.
• The stronger the acid, the weaker its conjugate base.
• Strong acids to know: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄.

Worked example

Acetic acid has pKa 4.76, so Ka is about 1.7 x 10^-5. Its conjugate base acetate has pKb = 14 - 4.76 = 9.24. A 0.1 M solution: [H⁺] is about sqrt(Ka x C) = sqrt(1.7 x 10^-6), about 1.3 x 10^-3 M, pH about 2.9.

What makes an acid stronger

Stability of the conjugate base: more electronegative atom, larger atom down a group, resonance, and electron-withdrawing groups nearby.

Common traps

• Reading a larger pKa as stronger. Larger pKa means weaker.
• Calling the conjugate base of a strong acid a strong base. It is negligibly basic.
• Using the weak acid shortcut for a strong acid. For a strong acid, [H⁺] equals the concentration.

One-minute recap

• Low pKa, high Ka, stronger acid.
• pKa + pKb = 14.
• Strong acid, spectator conjugate base.