Strong and weak acids and bases

All the way, or only a little

The strength of an acid depends on how far it ionizes in water. A strong acid splits completely into ions. A weak acid splits only partly; most of its molecules stay whole.

Mineral acids are generally strong. Hydrochloric acid ionizes completely, giving a large amount of H₃O⁺. Nitric acid ionizes in one step. Sulphuric acid has two hydrogens to give, so it ionizes in two steps: first to HSO₄⁻, then to SO₄²⁻.

Organic acids ionize only a little, so they are weak. The book gives acetic acid an ionization of 0.132%: only about 1.32 of every 1000 molecules split. Formic acid (1.50 M) is 1.06% ionized.

Bases work the same way. NaOH and KOH ionize completely, so they are strong bases. NH₄OH and Al(OH)₃ ionize only partly, so they are weak bases. Weak bases are used as antacids: they neutralize extra stomach acid without harming the throat or stomach.

Strong or weak is about how much splits into ions, not how much acid is in the bottle.

Key terms

Strong acid

Lots of H₃O⁺.

Weak acid

Little H₃O⁺.

Diprotic acid

Sulphuric acid.

Percent ionization

Measuring strength.

Antacid

Curing hyperacidity.

Short questions with model answers

1. Q1. Formic acid (1.50 M) is 1.06% ionized. Out of 1000 molecules, how many remain whole?

   • Ionized molecules
   • Subtract

   About 989 (the book prints 987; see the build report).

2. Q2. What is the difference between a strong acid and a concentrated acid? (Short question v, p. 104)

   • Strong
   • Concentrated

   Strength is the fraction that ionizes; concentration is the amount dissolved. A dilute HCl solution is still a strong acid.

Common mistakes

• ✗ Strong means concentrated.

  ✓ Strong means complete ionization; concentrated means a lot dissolved.

• ✗ Acetic acid is a strong acid because vinegar is sour.

  ✓ Only about 0.132% ionizes, so it is weak (p. 97).

• ✗ H₂SO₄ gives both H⁺ ions in one step.

  ✓ It ionizes in two steps (p. 97).

MCQs

1. 1. The strength of an acid depends on:

   1. (a) Its colour
   2. (b) The extent it ionizes in water
   3. (c) Its taste
   4. (d) The size of the bottle
   Show answer

   (b) p. 97.

2. 2. Which is a weak acid?

   1. (a) HCl
   2. (b) HNO₃
   3. (c) CH₃COOH
   4. (d) H₂SO₄
   Show answer

   (c) Organic acids ionize very little (p. 97).

3. 3. Which is a weak base?

   1. (a) NaOH
   2. (b) KOH
   3. (c) NH₄OH
   4. (d) None
   Show answer

   (c) p. 98: partial ionization.

4. 4. Sulphuric acid ionizes in:

   1. (a) One step
   2. (b) Two steps
   3. (c) Three steps
   4. (d) It does not ionize
   Show answer

   (b) It is diprotic (p. 97).

5. 5. Acetic acid is 0.132% ionized. Out of 1000 molecules, about how many split?

   1. (a) 132
   2. (b) 13.2
   3. (c) 1.32
   4. (d) 0.132
   Show answer

   (c) 0.132% of 1000 is 1.32 (p. 97).

Quick revision

• Strong acids and bases ionize completely; weak ones only partly.
• HCl, HNO₃, H₂SO₄, NaOH and KOH are strong; organic acids, NH₄OH and Al(OH)₃ are weak.
• H₂SO₄ is diprotic and ionizes in two steps.
• Strong is not the same as concentrated.