Theory of titration of very weak acids and bases MCQs With Answer

Theory of titration of very weak acids and bases MCQs With Answer
Understanding titration theory for very weak acids and bases is essential for B. Pharm students studying analytical chemistry, pharmacokinetics, and formulation science. This topic covers titration curves, buffer regions, half‑equivalence and equivalence points, pKa/pKb determination, conjugate salt hydrolysis, indicator selection, activity vs concentration effects, and practical calculation methods used in pharmaceutical analysis. Grasping how very weak acids or bases behave during neutralization helps predict pH-dependent drug solubility, stability, and absorption. These MCQs emphasize deeper reasoning, mathematical approaches, and real drug examples to build competence in laboratory titration and quality control. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What defines a “very weak acid” in the context of titration theory?

• An acid with a very small Ka (high pKa), showing negligible ionization in water
• An acid that reacts extremely slowly with bases
• An acid that has a Ka greater than 1
• An acid that forms insoluble salts with bases

Correct Answer: An acid with a very small Ka (high pKa), showing negligible ionization in water

Q2. During titration of a weak acid with a strong base, what is the pH at the half‑equivalence point?

• pH = pKa of the acid
• pH = pKb of the conjugate base
• pH = 7.00 always
• pH = 14 − pKa

Correct Answer: pH = pKa of the acid

Q3. For a very weak acid titrated with a strong base, how does the pH at the equivalence point compare to neutral pH?

• It is greater than 7 due to hydrolysis of the conjugate base
• It is always equal to 7
• It is less than 7 because acid remains
• It cannot be predicted without temperature data

Correct Answer: It is greater than 7 due to hydrolysis of the conjugate base

Q4. Which principle is most important when selecting an indicator for titration of a very weak acid with NaOH?

• Indicator transition range should overlap the expected equivalence pH
• Indicator must be structurally similar to the analyte
• Any indicator with pH range 3–10 will work equally well
• Indicator color must be fluorescent

Correct Answer: Indicator transition range should overlap the expected equivalence pH

Q5. Why does a buffer region appear during titration of a weak acid with a strong base?

• Because appreciable amounts of both weak acid and its conjugate base coexist before equivalence
• Because the titrant is impure
• Because the indicator forms a buffer
• Because temperature changes during titration

Correct Answer: Because appreciable amounts of both weak acid and its conjugate base coexist before equivalence

Q6. Which equation is most directly used to calculate pH in the buffer region of a weak acid titration?

• Henderson–Hasselbalch equation
• Arrhenius equation
• Van ‘t Hoff equation
• Beer–Lambert law

Correct Answer: Henderson–Hasselbalch equation

Q7. How does ionic strength of the solution affect titration calculations for very weak acids and bases?

• Ionic strength alters activity coefficients, so using concentrations instead of activities causes pH deviations
• Ionic strength has no effect; Ka is constant
• Increasing ionic strength always decreases pH by 1 unit
• Ionic strength only matters for redox titrations

Correct Answer: Ionic strength alters activity coefficients, so using concentrations instead of activities causes pH deviations

Q8. Compared with a strong acid, how does the initial pH of a solution of a very weak acid of equal concentration typically differ?

• The initial pH of the very weak acid is higher (less acidic)
• The initial pH is lower (more acidic)
• They have identical initial pH values
• Initial pH depends only on temperature, not acid strength

Correct Answer: The initial pH of the very weak acid is higher (less acidic)

Q9. When titrating a very weak base with a strong acid, what is the expected pH at equivalence?

• Less than 7 due to hydrolysis of the conjugate acid
• Equal to 7 because neutralization is complete
• Greater than 7 because base remains
• Exactly equal to pKb of the base

Correct Answer: Less than 7 due to hydrolysis of the conjugate acid

Q10. Which titrant is standard and most appropriate for titrating very weak acids in pharmaceutical labs?

• Standardized NaOH solution
• Dilute acetic acid
• Non‑aqueous alcohol
• Saturated sodium chloride

Correct Answer: Standardized NaOH solution

Q11. To calculate pH at equivalence for a weak acid titration, which value must you compute from Ka?

• The Kb of the conjugate base using Kb = Kw/Ka
• The molar mass of the acid
• The solubility product of the salt
• The oxidation potential of the acid

Correct Answer: The Kb of the conjugate base using Kb = Kw/Ka

Q12. Which of the following is a good example of a very weak acid relevant to pharmacy (high pKa)?

• Phenol (pKa ≈ 10)
• Hydrochloric acid (pKa < 0)
• Hydrogen peroxide (not an acid in this sense)
• Sodium hydroxide (a base)

Correct Answer: Phenol (pKa ≈ 10)

Q13. What is the best general strategy to choose an indicator for titration of a very weak base with a strong acid?

• Select an indicator whose transition range is near the predicted acidic equivalence pH
• Choose an indicator that changes color at pH 7 only
• Use any indicator that is soluble in organic solvents
• Always use phenolphthalein regardless of base strength

Correct Answer: Select an indicator whose transition range is near the predicted acidic equivalence pH

Q14. How does dilution of the analyte solution affect the equivalence point pH for a weak acid titration?

• Equivalence pH changes with concentration because hydrolysis depends on absolute concentration
• Equivalence pH is independent of dilution
• Dilution only affects color of the indicator
• Dilution converts weak acid into strong acid

Correct Answer: Equivalence pH changes with concentration because hydrolysis depends on absolute concentration

Q15. Which experimental titration feature allows direct experimental determination of pKa for a weak acid?

• The pH at half‑equivalence point
• The initial pH before titration
• The final pH well past equivalence
• The endpoint volume of titrant only

Correct Answer: The pH at half‑equivalence point

Q16. A very weak acid has Ka = 1.0 × 10−10. What is its pKa approximately?

• pKa ≈ 10
• pKa ≈ −10
• pKa ≈ 4
• pKa ≈ 0.1

Correct Answer: pKa ≈ 10

Q17. Why does the solution at equivalence become basic when titrating a very weak acid with a strong base?

• The conjugate base formed hydrolyzes water to produce OH− ions
• Excess CO2 from air dissolves and raises pH
• The strong base is not consumed at equivalence
• The indicator produces OH−

Correct Answer: The conjugate base formed hydrolyzes water to produce OH− ions

Q18. For titrations where the equivalence pH is high and the endpoint is broad, which detection method is generally more accurate?

• Potentiometric titration (pH meter)
• Colorimetric indicator only
• Visual turbidity measurement
• Mass spectrometry

Correct Answer: Potentiometric titration (pH meter)

Q19. The Henderson–Hasselbalch equation assumes which simplification?

• Activities can be approximated by concentrations or activity coefficients are near unity
• The titration is performed at absolute zero
• The acid is fully dissociated
• The solution contains only nonionic species

Correct Answer: Activities can be approximated by concentrations or activity coefficients are near unity

Q20. For a weak acid titration, which approximate relation can be used to estimate [OH−] at equivalence from Kb and salt concentration c?

• [OH−] ≈ sqrt(Kb × c_salt)
• [OH−] = Kb / c_salt
• [OH−] = Ka × c_salt
• [OH−] = c_salt / Kw

Correct Answer: [OH−] ≈ sqrt(Kb × c_salt)

Q21. How does the common‑ion effect influence the titration of a very weak acid when its conjugate base is present?

• The common ion suppresses acid ionization, shifting equilibrium and raising pH relative to pure acid
• The common ion increases ionization and lowers pH
• Common ion has no effect on acid ionization
• Common ion converts the acid into a salt precipitate

Correct Answer: The common ion suppresses acid ionization, shifting equilibrium and raising pH relative to pure acid

Q22. If equal volumes and concentrations of the same monoprotic acid are titrated with identical titrant, how do stoichiometric volumes at equivalence compare between a weak and a strong acid?

• They are the same because stoichiometry is identical
• Weak acid requires more titrant due to incomplete dissociation
• Strong acid requires more titrant because it is fully ionized
• They cannot be compared without indicator data

Correct Answer: They are the same because stoichiometry is identical

Q23. Which indicator would be least suitable for titrating a very weak acid whose equivalence pH is expected near 9.5?

• Methyl orange (transition 3.1–4.4)
• Phenolphthalein (transition 8.2–10.0)
• Bromothymol blue (transition 6.0–7.6)
• Thymolphthalein (transition 9.3–10.5)

Correct Answer: Methyl orange (transition 3.1–4.4)

Q24. How does increasing titrant concentration (while keeping analyte moles constant) affect the titration endpoint sharpness?

• Higher titrant concentration gives a steeper pH change at equivalence, easing endpoint detection
• Higher concentration always decreases accuracy and broadens the endpoint
• Titrant concentration has no effect on endpoint shape
• Endpoint becomes undetectable with stronger titrant

Correct Answer: Higher titrant concentration gives a steeper pH change at equivalence, easing endpoint detection

Q25. Besides the half‑equivalence method, which analytical plot is useful to determine pKa from titration data for very weak acids?

• Gran plot
• Lineweaver–Burk plot
• Schild plot
• Tauc plot

Correct Answer: Gran plot

Q26. Why is temperature control important when titrating very weak acids/bases in pharmaceutical labs?

• Ka/Kb values are temperature dependent, altering pH and equivalence point
• Temperature only affects color of indicators but not pH
• Titration reactions are inert to temperature changes
• Temperature changes precipitate the titrant

Correct Answer: Ka/Kb values are temperature dependent, altering pH and equivalence point

Q27. What stoichiometric condition defines the equivalence point in any acid‑base titration?

• Moles of acid neutralized equals moles of base added (stoichiometric equivalence)
• pH equals 7.00
• Titrant volume is maximum
• Indicator is colorless

Correct Answer: Moles of acid neutralized equals moles of base added (stoichiometric equivalence)

Q28. For a diprotic very weak acid, when will the two equivalence points be clearly resolved?

• When the difference between the two pKa values is greater than about 2
• When both pKa values are equal
• Only when titrant concentration is extremely low
• Diprotic acids never show two equivalence points

Correct Answer: When the difference between the two pKa values is greater than about 2

Q29. What is a main advantage of conductometric titration for very weak acids/bases in pharmaceutical samples?

• It is indicator‑free and useful for colored or turbid solutions where visual end‑points fail
• It always gives pKa directly without calculation
• It requires no calibration
• It measures only the ionic strength, not endpoint

Correct Answer: It is indicator‑free and useful for colored or turbid solutions where visual end‑points fail

Q30. Why is mastery of titration theory for very weak acids and bases clinically and pharmaceutically important for B. Pharm students?

• Because drug ionization, solubility, stability, formulation pH and bioavailability depend on accurate pKa and titration analysis
• It is only a theoretical exercise with no real‑world application
• Because most drugs are strong acids and bases
• Only for passing exams; it has no impact on drug design

Correct Answer: Because drug ionization, solubility, stability, formulation pH and bioavailability depend on accurate pKa and titration analysis

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com