AP Chemistry 8.2: Strong Acids and Bases: pH and pOH

The word strong has a very specific meaning in acid-base chemistry.

• A strong acid ionizes essentially completely in water, while a strong base dissociates essentially completely in water.
• This does not mean “strong” and “concentrated” are the same thing.

Strength describes the extent of ionization.

Concentration describes how much substance is present per volume of solution.

A dilute strong acid can have a higher pH than a concentrated weak acid depending on the concentrations involved.

The commonly memorized strong acids include:

• HCl
• HBr
• HI
• HNO₃
• HClO₄
• H₂SO₄, especially its first proton in introductory AP treatment.

Because strong acids ionize essentially completely, you normally do not construct a Ka ICE table for a simple strong-acid calculation.

• For example:
• HCl + H₂O → H₃O⁺ + Cl⁻
• If:
• [HCl] = 0.0100 M
• then approximately:
• [H₃O⁺] = 0.0100 M
• Therefore:
• pH = −log(0.0100)
• pH = 2.000

Group 1 metal hydroxides such as:

• LiOH, NaOH, KOH
• are strong bases.

Certain soluble Group 2 hydroxides can also behave as strong bases.

• For example:
• NaOH → Na⁺ + OH⁻
• If:
• [NaOH] = 0.0200 M
• then:
• [OH⁻] = 0.0200 M
• Calculate:
• pOH = −log(0.0200)
• pOH ≈ 1.699
• Then:
• pH = 14.00 − 1.699
• pH ≈ 12.301

Suppose a strong base produces more than one OH⁻ per formula unit.

• For example:
• Ba(OH)₂ → Ba²⁺ + 2OH⁻
• If:
• [Ba(OH)₂] = 0.010 M
• then:
• [OH⁻] = 2(0.010)
• [OH⁻] = 0.020 M

Do not automatically set hydroxide concentration equal to the original base concentration.

Always use the balanced dissociation equation.

Suppose you compare:

• 0.00010 M HCl
• and:
• 1.0 M weak acid.
• HCl is still the stronger acid because it ionizes much more completely.
• However, the concentrated weak-acid solution may still contain more H₃O⁺ overall.
• Therefore:
• acid strength ≠ acid concentration
• This distinction appears frequently in AP questions.

A particle diagram of a strong acid should show almost entirely:

• H₃O⁺ and conjugate-base ions
• rather than intact acid molecules.

A weak acid should show:

• mostly intact HA molecules
• with smaller amounts of:
• H₃O⁺ and A⁻
• This is one way AP Chemistry connects microscopic particle representations to equilibrium.

Do not construct a weak-acid ICE table for a normal strong-acid problem.

Do not confuse strong with concentrated.

For bases containing multiple hydroxides, account for the stoichiometric number of OH⁻ ions.

If you calculate OH⁻ concentration, remember that you have pOH, not pH.

Strong acid:

• concentration → [H₃O⁺] → pH
• Strong base:
• concentration → [OH⁻] → pOH → pH

Remember:

• strong ≠ concentrated
• and:
• weak ≠ dilute