Sodium chloride dissolves in water. What pH do you expect, and why?
ASlightly basic — Cl⁻ accepts protons from water, raising [OH⁻]
BNeutral (pH ≈ 7) — Cl⁻ is the conjugate base of a strong acid and does not hydrolyze
CSlightly acidic — Na⁺ acts as a weak Lewis acid in solution
DBasic — both Na⁺ and Cl⁻ hydrolyze to produce OH⁻
Cl⁻ is the conjugate base of HCl, a strong acid. Because HCl fully ionizes, its Ka is enormous, and by Ka × Kb = Kw, Kb for Cl⁻ ≈ 10⁻¹⁴/∞ ≈ 0. Cl⁻ effectively cannot accept a proton from water. Option A is the classic misconception — students assume all anions are bases, but only conjugate bases of weak acids have a non-negligible Kb.
Question 2 Multiple Choice
The Ka of acetic acid is 1.8 × 10⁻⁵. What is the Kb of acetate (CH₃COO⁻)?
A1.8 × 10⁻⁵ — the conjugate base has the same equilibrium constant as the acid
B5.6 × 10⁻¹⁰ — calculated as Kw / Ka = 1.0 × 10⁻¹⁴ / 1.8 × 10⁻⁵
C1.0 × 10⁻¹⁴ — Kb of any base equals Kw
D5.6 × 10⁴ — the reciprocal of Ka
Ka × Kb = Kw links every conjugate acid-base pair. Rearranging: Kb = Kw / Ka = (1.0 × 10⁻¹⁴) / (1.8 × 10⁻⁵) = 5.6 × 10⁻¹⁰. Note that a stronger acid (larger Ka) produces a weaker conjugate base (smaller Kb) — the relationship is inverse, not equal.
Question 3 True / False
A weak base at 0.1 M produces a more basic solution than a strong base at 0.1 M.
TTrue
FFalse
Answer: False
A strong base fully ionizes, so 0.1 M NaOH produces exactly 0.1 M OH⁻ (pOH = 1, pH = 13). A weak base only partially ionizes — 0.1 M ammonia (Kb ≈ 1.8 × 10⁻⁵) produces about 1.3 × 10⁻³ M OH⁻ (pH ≈ 11.1). Weak bases, by definition, produce far fewer hydroxide ions than strong bases at the same concentration.
Question 4 True / False
For any conjugate acid-base pair, if Ka increases (the acid becomes stronger), Kb must decrease.
TTrue
FFalse
Answer: True
Because Ka × Kb = Kw = 1.0 × 10⁻¹⁴ (at 25°C) for any conjugate pair, Ka and Kb have a fixed inverse product. If Ka doubles, Kb must halve. This is the quantitative expression of the qualitative rule: a stronger acid has a weaker conjugate base.
Question 5 Short Answer
Why is the relationship pH + pOH = 14 only valid at 25°C? What changes at other temperatures?
Think about your answer, then reveal below.
Model answer: The relationship comes from Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C, giving pH + pOH = −log(Kw) = 14. Kw is temperature-dependent — at 37°C (body temperature), Kw ≈ 2.4 × 10⁻¹⁴, so pH + pOH ≈ 13.62. Neutral pH (where [H⁺] = [OH⁻]) is also not 7 at other temperatures.
Students who memorize 'pH + pOH = 14' without understanding its origin make errors in problems involving elevated temperatures. The 14 is −log(Kw), and Kw increases with temperature because water autoionization is endothermic. At higher temperatures, both the sum and the neutral pH shift.