Theory of titration of weak acids and bases MCQs With Answer

Theory of titration of weak acids and bases MCQs With Answer

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Theory of titration of weak acids and bases MCQs With Answer

Theory of titration of weak acids and bases is a core topic in pharmaceutical analysis for B.Pharm students. It covers titration curves, equivalence point, half-equivalence point, pKa/pKb relationships, buffer regions, indicator selection, and volumetric calculations. Understanding acid–base equilibria, Henderson–Hasselbalch equation, salt hydrolysis, ionic strength effects, and titration methodology prepares students for accurate assay of drugs and excipients. This set of focused, application-oriented MCQs reinforces concepts such as pH changes during titration, buffer capacity, endpoint versus equivalence, and practical choices of indicators and standard solutions. Clear grasp of these principles aids quality control, formulation, and analytical problem solving. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary purpose of titration in the analysis of weak acids or bases?

  • To determine the equilibrium constant Ka or Kb directly
  • To determine the concentration of an unknown acid or base by reaction with a standard solution
  • To measure the ionic strength of a solution
  • To change the pH of a solution for formulation purposes

Correct Answer: To determine the concentration of an unknown acid or base by reaction with a standard solution

Q2. In the titration of a weak acid with a strong base, the pH at the equivalence point is typically:

  • Less than 7
  • Equal to 7
  • Greater than 7
  • Undefined

Correct Answer: Greater than 7

Q3. At the half-equivalence point during titration of a weak acid with a strong base, which relation holds true?

  • pH = pKb
  • pH = pKa
  • pH = 7
  • pH = equivalence point pH

Correct Answer: pH = pKa

Q4. Which equation best describes the pH of a buffer composed of a weak acid HA and its conjugate base A-?

  • pH = pKb + log([HA]/[A-])
  • pH = pKa – log([A-]/[HA])
  • pH = pKa + log([A-]/[HA])
  • pH = pKw – pKa

Correct Answer: pH = pKa + log([A-]/[HA])

Q5. When selecting an indicator for titration of a weak acid with a strong base, the best criterion is:

  • Indicator with transition range near initial pH
  • Indicator with transition range overlapping the steep portion of the titration curve around the equivalence point
  • Indicator with the widest possible color range
  • Indicator that is fluorescent

Correct Answer: Indicator with transition range overlapping the steep portion of the titration curve around the equivalence point

Q6. Which feature distinguishes the titration curve of a weak acid with a strong base from that of a strong acid with a strong base?

  • The weak acid curve shows no equivalence point
  • The weak acid curve has a buffer region and an equivalence pH > 7
  • The weak acid curve is always steeper at equivalence
  • The weak acid curve starts at pH 1

Correct Answer: The weak acid curve has a buffer region and an equivalence pH > 7

Q7. During titration of a weak base with a strong acid, the equivalence point pH is typically:

  • Greater than 7
  • Equal to 7
  • Less than 7
  • Exactly pKb

Correct Answer: Less than 7

Q8. Titration of a weak acid with a weak base typically shows which experimental difficulty?

  • Very sharp equivalence jump, making endpoint detection trivial
  • No change in pH at all
  • Small pH change near equivalence, making endpoint detection difficult
  • The equivalence point is always at pH 7

Correct Answer: Small pH change near equivalence, making endpoint detection difficult

Q9. The half-equivalence point during a titration is defined as the point where:

  • The pH equals the equivalence pH
  • Half of the titrant volume has been added
  • Half of the analyte has been neutralized and [HA] = [A-]
  • The pOH equals pKb

Correct Answer: Half of the analyte has been neutralized and [HA] = [A-]

Q10. If an acid has Ka = 1.0 × 10^-5, what is its pKa (approximately)?

  • 5.0
  • 9.0
  • 1.0
  • 10.0

Correct Answer: 5.0

Q11. Why does ionic strength of a solution affect titration results for weak acids and bases?

  • Ionic strength changes the color of indicators
  • Ionic strength affects activity coefficients, altering effective concentrations and pH
  • Ionic strength changes electrode material
  • Ionic strength has no effect on titration

Correct Answer: Ionic strength affects activity coefficients, altering effective concentrations and pH

Q12. The common ion effect in a titration system typically causes:

  • Increased ionization of the weak acid or base
  • Suppression of ionization of the weak acid or base, shifting equilibrium
  • No change in buffer capacity
  • Spontaneous precipitation

Correct Answer: Suppression of ionization of the weak acid or base, shifting equilibrium

Q13. Which statement describes salt hydrolysis relevant to titration of a weak acid with strong base?

  • The salt of a weak acid and strong base yields an acidic solution
  • The salt does not affect pH
  • The salt of a weak acid and strong base yields a basic solution due to hydrolysis of the conjugate base
  • The salt of a weak acid and strong base is always neutral

Correct Answer: The salt of a weak acid and strong base yields a basic solution due to hydrolysis of the conjugate base

Q14. For a diprotic acid with two ionizable protons, the titration will normally show:

  • One equivalence point only
  • Two equivalence points corresponding to successive deprotonations
  • Equivalence points equal to pKa values
  • No buffer regions

Correct Answer: Two equivalence points corresponding to successive deprotonations

Q15. What is the purpose of a Gran plot in titrations of weak acids?

  • To visualize buffer capacity
  • To linearize titration data and accurately determine the equivalence point
  • To select the appropriate indicator color
  • To determine ionic strength

Correct Answer: To linearize titration data and accurately determine the equivalence point

Q16. Potassium hydrogen phthalate (KHP) is commonly used to standardize NaOH because KHP is:

  • Highly volatile
  • Monoprotic, stable, and pure solid with known assay
  • A strong base
  • An indicator

Correct Answer: Monoprotic, stable, and pure solid with known assay

Q17. Phenolphthalein is often chosen as an indicator for weak acid–strong base titrations because its transition range is approximately:

  • 3.0–4.5
  • 4.5–6.0
  • 6.8–7.2
  • 8.2–10.0

Correct Answer: 8.2–10.0

Q18. If the measured pH at the half-equivalence point of an acetic acid titration is 4.76, what is the pKa of acetic acid?

  • 1.0
  • 4.76
  • 7.0
  • 9.24

Correct Answer: 4.76

Q19. Buffer capacity is maximal when:

  • pH = 0
  • [A-] = [HA] and pH = pKa
  • Only strong acid is present
  • Only salt is present

Correct Answer: [A-] = [HA] and pH = pKa

Q20. Which factor most strongly affects the steepness of the pH change at the equivalence point?

  • Color of the indicator
  • Concentrations of acid and titrant and buffer capacity
  • Ambient room temperature only
  • Shape of the titration flask

Correct Answer: Concentrations of acid and titrant and buffer capacity

Q21. Compared to an indicator, a pH meter for determining endpoint in weak acid–base titrations is usually:

  • Less accurate and less precise
  • More accurate and provides continuous pH measurement
  • Only suitable for strong acid titrations
  • Unrelated to titration

Correct Answer: More accurate and provides continuous pH measurement

Q22. In titration terminology, the difference between endpoint and equivalence point is that:

  • Endpoint is theoretical; equivalence is observed
  • Endpoint is observed (indicator change); equivalence is the stoichiometric point
  • They are always identical
  • Endpoint is when pH = 7 and equivalence when volumes are equal

Correct Answer: Endpoint is observed (indicator change); equivalence is the stoichiometric point

Q23. The pH at the equivalence point when titrating a weak acid with a weak base generally:

  • Is always 7
  • Depends on the relative Ka and Kb values of the acid and base
  • Is always greater than 14
  • Is equal to the initial pH

Correct Answer: Depends on the relative Ka and Kb values of the acid and base

Q24. Acetate ion in the solution formed at the equivalence point of acetic acid titration with NaOH causes the solution to be basic because:

  • Acetate is a strong acid
  • Acetate undergoes hydrolysis producing OH-
  • Acetate produces H+ from water
  • NaOH remains unreacted

Correct Answer: Acetate undergoes hydrolysis producing OH-

Q25. For a conjugate acid–base pair at 25°C, the relationship between pKa and pKb is approximately:

  • pKa × pKb = 14
  • pKa + pKb = 14
  • pKa = pKb
  • pKa – pKb = Kw

Correct Answer: pKa + pKb = 14

Q26. The initial pH of a 0.01 M weak acid solution is generally:

  • Lower (more acidic) than that of a strong acid of the same concentration
  • Higher (less acidic) than that of a strong acid of the same concentration
  • Always neutral
  • Equal to the equivalence point pH

Correct Answer: Higher (less acidic) than that of a strong acid of the same concentration

Q27. The observed color change of an indicator at the endpoint is a result of:

  • A chemical reaction between indicator and titrant that changes pH
  • A sudden temperature change
  • Indicator changing form as pH crosses its transition range
  • Precipitation of the analyte

Correct Answer: Indicator changing form as pH crosses its transition range

Q28. If a weak base has pKb = 4.0, the pKa of its conjugate acid (at 25°C) is approximately:

  • 20.0
  • 4.0
  • 10.0
  • 14.0

Correct Answer: 10.0

Q29. For a polyprotic acid, the pH at the first half-equivalence point corresponds to:

  • pH = pKa1 where the first proton is half-neutralized
  • pH = pKa2 where the second proton is fully neutralized
  • pH = equivalence point
  • pH = pKw

Correct Answer: pH = pKa1 where the first proton is half-neutralized

Q30. The buffer region in a titration curve is characterized by which of the following?

  • Rapid, steep change in pH with addition of titrant
  • Little change in pH on addition of moderate amounts of titrant due to conjugate acid/base pair
  • pH fixed at 7.00 throughout
  • Where precipitation occurs

Correct Answer: Little change in pH on addition of moderate amounts of titrant due to conjugate acid/base pair

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