You dissolve sodium acetate (NaC₂H₃O₂) in water. Without calculating, which best describes the resulting solution?
AAcidic, because sodium ions are weakly acidic
BBasic, because acetate ions accept a proton from water to form OH⁻
CNeutral, because Na⁺ and C₂H₃O₂⁻ charges cancel
DAcidic, because acetic acid is a weak acid
Sodium ion is a spectator (conjugate of the strong base NaOH). Acetate is the conjugate base of the weak acid acetic acid; it undergoes hydrolysis: C₂H₃O₂⁻ + H₂O → HC₂H₃O₂ + OH⁻, making the solution basic. This is the salt hydrolysis concept — a salt of a weak acid and strong base produces a basic solution.
Question 2 True / False
Adding a small amount of strong acid to a buffer solution does not change its pH at most.
TTrue
FFalse
Answer: False
Buffers resist pH change but do not prevent it entirely. The Henderson-Hasselbalch equation shows that adding acid consumes conjugate base ([A⁻] decreases, [HA] increases), shifting the ratio and slightly lowering pH. The buffer absorbs the shock — it just cannot prevent every change. A buffer is overwhelmed entirely when one component is exhausted.
Question 3 Short Answer
Acetic acid has Ka = 1.8 × 10⁻⁵. What is the Kb for acetate ion, and what relationship makes this calculation possible?
Think about your answer, then reveal below.
Model answer: Kb = Kw / Ka = (1.0 × 10⁻¹⁴) / (1.8 × 10⁻⁵) ≈ 5.6 × 10⁻¹⁰. The relationship Ka × Kb = Kw holds for any conjugate acid-base pair.
For conjugate acid-base pairs, Ka × Kb = Kw because the two equilibria — HA ⇌ H⁺ + A⁻ and A⁻ + H₂O ⇌ HA + OH⁻ — sum to the water autoionization equilibrium H₂O ⇌ H⁺ + OH⁻ with constant Kw. This relationship is a direct consequence of Hess's law applied to equilibria.