Questions: Acid–Base Titrations and Buffer Systems

Question 1

Acetic acid (a weak acid) is titrated with sodium hydroxide (a strong base). What is the expected pH at the equivalence point?

• A. Exactly 7.00, because equal molar amounts of acid and base have been combined
• B. Below 7, because excess acetic acid remains at the equivalence point
• C. Above 7, because the sodium acetate produced hydrolyzes water to generate hydroxide ions
• D. Exactly 7 for all monoprotic acids regardless of their strength

Answer: (C)

At the equivalence point of a weak acid–strong base titration, all the weak acid has been converted to its conjugate base (sodium acetate). Acetate ion is itself a weak base that hydrolyzes water: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻. This produces hydroxide, making the solution basic — pH > 7. Only for strong acid–strong base titrations does the equivalence point fall at pH 7, because neither product (Na⁺ nor Cl⁻) affects water equilibrium. The common mistake is assuming neutralization always yields pH 7.

Question 2

During a weak acid–strong base titration, the pH equals 4.75 at the half-equivalence point. What does this tell you about the acid?

• A. The molar mass of the acid is 4.75 g/mol
• B. The concentration of the acid solution is 4.75 M
• C. The pKa of the acid is 4.75, because at half-equivalence [HA] = [A⁻] and the Henderson–Hasselbalch equation reduces to pH = pKa
• D. The titration is 47.5% complete at this measurement

Answer: (C)

At the half-equivalence point, exactly half the weak acid has been converted to its conjugate base, so [HA] = [A⁻]. Substituting into Henderson–Hasselbalch: pH = pKa + log([A⁻]/[HA]) = pKa + log(1) = pKa + 0 = pKa. This makes acid–base titration a practical tool for determining pKa values experimentally — simply read the pH at the half-equivalence point from the titration curve. The buffer region near this point is flat precisely because pH ≈ pKa throughout.

Question 3

For a strong acid–strong base titration, the equivalence point occurs at pH 7.

• T. True
• F. False

Answer: True

When a strong acid (e.g., HCl) is fully neutralized by a strong base (e.g., NaOH), the products are a neutral salt (NaCl) and water. NaCl dissociates completely into Na⁺ and Cl⁻, neither of which undergoes hydrolysis or perturbs water's autoionization equilibrium. Therefore, the solution at the equivalence point is effectively pure salt water at pH 7. This is true specifically for strong acid–strong base pairs; weak acid or weak base systems produce non-neutral equivalence points.

Question 4

Any indicator that changes color in the basic pH range can be used for any acid–base titration.

• T. True
• F. False

Answer: False

An indicator must change color within the steep portion of the titration curve — the narrow range where a tiny volume of titrant causes a large pH swing. That steep region differs by titration type: for strong acid–strong base it spans roughly pH 4–10; for weak acid–strong base it is narrower and shifted basic (approximately pH 7–11). Using an indicator that transitions outside the steep region — even if it is technically 'in the basic range' — means the color change occurs before or after the true equivalence point, introducing systematic error.

Question 5

Why does the pH at the equivalence point of a weak acid–strong base titration exceed 7, even though equal moles of acid and base were combined?

Model answer: At the equivalence point, all the weak acid has been converted to its conjugate base. Unlike the neutral products of a strong acid–strong base reaction, a weak acid's conjugate base is itself a weak base that reacts with water: A⁻ + H₂O ⇌ HA + OH⁻. This hydrolysis generates hydroxide ions, making the solution basic. The pH above 7 is not a failure of the stoichiometry — it reflects the incomplete ionization character of the original weak acid: its conjugate base retains enough basicity to raise the solution's pH.

This follows directly from the definition of a weak acid: a weak acid does not fully donate its proton to water, so its conjugate base retains the capacity to accept protons from water. The stronger the original acid (higher Ka), the weaker its conjugate base, and the closer the equivalence point pH approaches 7. For a truly strong acid, the conjugate base has negligible basicity — hence pH 7 at equivalence. Every equivalence point pH is determined by the species present in solution at that point, not by a universal neutralization rule.