Sorensen’s pH scale MCQs With Answer

Sorensen’s pH scale MCQs With Answer

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Sorensen’s pH scale MCQs With Answer offers B. Pharm students a focused review of pH fundamentals critical to drug formulation, stability, and analysis. This concise guide covers Sorensen’s definition of pH, hydrogen ion activity vs concentration, Henderson-Hasselbalch applications, buffer capacity, indicator selection, pH meter calibration, temperature effects, and pharmaceutical examples. These Sorensen’s pH scale MCQs emphasize practical calculations, buffer design for dosage forms, and interpretation of pH-related experimental data. Clear, exam-oriented questions help reinforce acid-base concepts and analytical techniques used in pharmaceutics. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the formal definition of pH according to Sorensen’s pH scale?

  • Negative logarithm of hydrogen ion concentration (-log[H+])
  • Negative logarithm of hydrogen ion activity (-log aH+)
  • Reciprocal of hydrogen ion concentration (1/[H+])
  • Logarithm of hydroxide ion concentration (log[OH-])

Correct Answer: Negative logarithm of hydrogen ion activity (-log aH+)

Q2. Who introduced the pH scale now commonly attributed to Sorensen?

  • Arrhenius
  • Sorensen (S.P.L. Sørensen)
  • Brønsted and Lowry
  • Henderson

Correct Answer: Sorensen (S.P.L. Sørensen)

Q3. At 25°C, what is the approximate numerical relationship between pH and pOH?

  • pH × pOH = 14
  • pH + pOH = 14
  • pH – pOH = 14
  • pH / pOH = 14

Correct Answer: pH + pOH = 14

Q4. Which equation relates pH, pKa, and the ratio of conjugate base to acid?

  • Nernst equation
  • Henderson-Hasselbalch equation
  • Van’t Hoff equation
  • Arrhenius equation

Correct Answer: Henderson-Hasselbalch equation

Q5. The Henderson-Hasselbalch equation is written as pH = pKa + log([A-]/[HA]). For a buffer where [A-] = [HA], the pH equals:

  • pKa + 1
  • pKa – 1
  • pKa
  • 14 – pKa

Correct Answer: pKa

Q6. Buffer capacity is best described as:

  • The pH value of a solution
  • The amount of acid or base needed to change pH by one unit
  • The solubility of a drug in a buffer
  • The ionic strength of the buffer

Correct Answer: The amount of acid or base needed to change pH by one unit

Q7. Which factor most strongly affects the pH measured by a glass electrode?

  • Color of the solution
  • Electrode potential dependent on hydrogen ion activity
  • Presence of undissociated molecules only
  • Magnetic field around the electrode

Correct Answer: Electrode potential dependent on hydrogen ion activity

Q8. Standard calibration of a pH meter typically uses which buffer values?

  • pH 1 and pH 13
  • pH 4, pH 7, and pH 10
  • pH 5 and pH 9 only
  • pH 6.5 and pH 8.5 only

Correct Answer: pH 4, pH 7, and pH 10

Q9. The Nernst slope for a single-proton change at 25°C is approximately:

  • 2.303 RT/F ≈ 59.16 mV per pH unit
  • 1 mV per pH unit
  • 100 mV per pH unit
  • 0 mV per pH unit

Correct Answer: 2.303 RT/F ≈ 59.16 mV per pH unit

Q10. Which statement about activity versus concentration is true in the context of Sorensen’s pH?

  • Activity equals concentration always
  • Activity corrects concentration for ionic interactions
  • Activity is irrelevant for pH measurements
  • Activity is only used for non-aqueous solvents

Correct Answer: Activity corrects concentration for ionic interactions

Q11. pKw is the ion product of water. At 25°C pKw equals approximately:

  • 7
  • 14
  • 1 × 10^-14
  • 0.1

Correct Answer: 14

Q12. If a solution has [H+] = 1 × 10^-3 M, its pH is:

  • 3
  • 11
  • 0.001
  • 7

Correct Answer: 3

Q13. For a weak acid HA with Ka = 1 × 10^-5 and initial concentration 0.1 M, which approach gives approximate pH?

  • Use strong acid assumption
  • Use Ka = [H+][A-]/[HA] and x approximation
  • Assume complete dissociation
  • Use pKw directly

Correct Answer: Use Ka = [H+][A-]/[HA] and x approximation

Q14. A buffer is most effective when the pH is:

  • Far from the pKa (±3 units)
  • Within ±1 unit of the pKa
  • Exactly equal to 14
  • Exactly neutral at pH 7 only

Correct Answer: Within ±1 unit of the pKa

Q15. In pharmaceutical formulations, controlling pH is important because:

  • pH has no effect on drug stability
  • pH affects solubility, stability, and permeability
  • Only aesthetics depend on pH
  • pH determines particle size exclusively

Correct Answer: pH affects solubility, stability, and permeability

Q16. Which indicator would be suitable for detecting pH change near pH 8.2?

  • Phenol red (pKa ~7.9)
  • Methyl orange (pKa ~3.7)
  • Methylene blue
  • Litmus only at pH 1

Correct Answer: Phenol red (pKa ~7.9)

Q17. Temperature affects pH measurements because:

  • Ion product of water (pKw) is temperature dependent
  • Glass electrode becomes insensitive at all temperatures
  • Henderson-Hasselbalch equation is invalid at different temperatures
  • Activity coefficients are constant with temperature

Correct Answer: Ion product of water (pKw) is temperature dependent

Q18. When preparing a phosphate buffer for parenteral use, which species pair is useful?

  • H2PO4- / HPO4^2-
  • HCl / Cl-
  • NH3 / NH4+
  • CH3COOH / CH3COOCH3

Correct Answer: H2PO4- / HPO4^2-

Q19. If a drug’s stability increases at pH 3, formulation scientists should:

  • Design a buffer around pH 3
  • Always use pH 7 for safety
  • Avoid any buffering to allow pH drift
  • Use only strong base to maintain stability

Correct Answer: Design a buffer around pH 3

Q20. Which statement about the glass electrode response is correct?

  • Response is proportional to hydroxide concentration only
  • Potential difference arises from hydrogen ion exchange at the glass membrane
  • Electrode measures total ionic strength directly
  • Glass electrodes require no calibration

Correct Answer: Potential difference arises from hydrogen ion exchange at the glass membrane

Q21. Activity coefficients become significant in pH calculations when:

  • Solutions are extremely dilute (<10^-6 M)
  • Ionic strength is high and concentrations are moderate to high
  • Only non-electrolytes are present
  • The temperature is exactly 25°C

Correct Answer: Ionic strength is high and concentrations are moderate to high

Q22. The primary reason Sorensen emphasized activity in pH definition was:

  • To complicate calculations
  • To account for ionic interactions changing effective hydrogen ion concentration
  • Because concentration units were not available then
  • To avoid using the logarithm

Correct Answer: To account for ionic interactions changing effective hydrogen ion concentration

Q23. For a solution of 0.01 M HCl, pH is:

  • 2
  • 0.01
  • 12
  • 7

Correct Answer: 2

Q24. When mixing acid and conjugate base to create a buffer, the buffer pH depends mainly on:

  • Total volume only
  • The ratio of conjugate base to acid
  • The color of reagents
  • The atomic weight of the acid

Correct Answer: The ratio of conjugate base to acid

Q25. A drug with pKa = 4.5 will be mostly ionized at pH:

  • 2.5
  • 4.5
  • 6.5
  • 4.6

Correct Answer: 6.5

Q26. For a monoprotic weak acid, which parameter directly increases buffer capacity near pKa?

  • Lower total concentration of acid and base
  • Higher total concentration of acid and base
  • Using an inert gas
  • Reducing ionic strength to zero

Correct Answer: Higher total concentration of acid and base

Q27. In practice, pH of biological fluids is controlled by:

  • Only strong acids
  • Buffer systems like bicarbonate, phosphate, and proteins
  • Adding pure water
  • Magnetic stirring

Correct Answer: Buffer systems like bicarbonate, phosphate, and proteins

Q28. If pH meter calibration shows a slope significantly different from theoretical Nernst slope, this indicates:

  • Perfect electrode performance
  • Possible electrode aging, contamination, or temperature mismatch
  • That pH is exactly neutral
  • That buffers used were pure water

Correct Answer: Possible electrode aging, contamination, or temperature mismatch

Q29. Which of the following best describes the buffer range for an acid with pKa = 5?

  • pH 2–3
  • pH 4–6
  • pH 7–9
  • pH 9–11

Correct Answer: pH 4–6

Q30. What happens to the pH of pure water as temperature increases from 25°C to 50°C?

  • pH increases to 9
  • Neutral pH decreases because pKw decreases, so pH < 7
  • pH remains exactly 7 always
  • Water becomes basic automatically

Correct Answer: Neutral pH decreases because pKw decreases, so pH < 7

Q31. Which calculation method is appropriate for determining pH of a weak base solution?

  • Treat base as fully dissociated
  • Use Kb expression and solve for [OH-]
  • Use pKw only
  • Use only molar mass

Correct Answer: Use Kb expression and solve for [OH-]

Q32. Why are pH and buffer considerations critical in lyophilized (freeze-dried) formulations?

  • They determine cake color only
  • They influence protein stability, aggregation, and degradation pathways
  • pH does not affect lyophilized products
  • Buffers evaporate during lyophilization making pH irrelevant

Correct Answer: They influence protein stability, aggregation, and degradation pathways

Q33. An ampholyte (zwitterion) like glycine has multiple pKa values. The isoelectric point (pI) is:

  • Sum of all pKa values
  • Average of pKa values around the neutral species
  • Always equal to 7
  • Difference between pKa values

Correct Answer: Average of pKa values around the neutral species

Q34. In titration of a weak acid with strong base, the equivalence point pH is:

  • Less than 7
  • Exactly 7
  • Greater than 7
  • Undefined

Correct Answer: Greater than 7

Q35. Which is a correct practical step to minimize junction potential errors during pH measurement?

  • Use a clean reference filling solution and proper salt bridge
  • Use distilled water as filling solution only
  • Keep electrode uncleaned to preserve response
  • Measure without calibration

Correct Answer: Use a clean reference filling solution and proper salt bridge

Q36. For two buffers of equal ionic strength, the one with higher total concentration will have:

  • Lower buffer capacity
  • Higher buffer capacity
  • The same buffer capacity always
  • No buffering effect

Correct Answer: Higher buffer capacity

Q37. Sorensen’s original work emphasized pH as a measure of:

  • Hydrogen ion concentration only
  • Hydrogen ion activity reflecting chemical reactivity
  • Electrical conductivity exclusively
  • Color change of indicators only

Correct Answer: Hydrogen ion activity reflecting chemical reactivity

Q38. Which buffer would be best for maintaining pH near 3.8 in an oral solution?

  • Acetate buffer (acetic acid / acetate)
  • Phosphate buffer (pKa around 7.2)
  • Tris buffer (pKa ~8.1)
  • Bicarbonate buffer (pKa 10.3)

Correct Answer: Acetate buffer (acetic acid / acetate)

Q39. In drug dissolution testing, pH of the dissolution medium affects:

  • Only the apparatus speed
  • Drug solubility and dissolution rate
  • Tablet weight only
  • Temperature but not the drug

Correct Answer: Drug solubility and dissolution rate

Q40. Which of the following is NOT a reason to control pH in parenteral formulations?

  • Prevent pain upon injection
  • Enhance drug stability
  • Ensure isotonicity solely
  • Prevent precipitation of drug

Correct Answer: Ensure isotonicity solely

Q41. The formal pH of a buffer solution can be calculated using:

  • Henderson-Hasselbalch using activities if available
  • Only indicator color without calculation
  • Mass balance ignoring dissociation
  • Boiling point elevation data

Correct Answer: Henderson-Hasselbalch using activities if available

Q42. Which laboratory practice improves accuracy of pH measurements in viscous pharmaceutical solutions?

  • Use an unclean electrode to avoid damage
  • Use a specialized electrode (e.g., spear or flat surface) and proper stirring
  • Measure without temperature control
  • Dilute sample with distilled water indiscriminately

Correct Answer: Use a specialized electrode (e.g., spear or flat surface) and proper stirring

Q43. For a weak acid with pKa 6.0, at pH 5.0 the ratio [A-]/[HA] is approximately:

  • 10:1
  • 1:10
  • 1:1
  • 100:1

Correct Answer: 1:10

Q44. Why might ionic strength adjustment be used when calibrating pH electrodes for pharmaceutical samples?

  • To make solutions non-conductive
  • To match matrix effects and reduce activity coefficient differences
  • To change the color of the buffer
  • To sterilize the electrodes

Correct Answer: To match matrix effects and reduce activity coefficient differences

Q45. A buffer prepared from acetic acid and sodium acetate will resist pH changes caused by:

  • Addition of small amounts of strong acid or base
  • Large additions of pure water only
  • Addition of insoluble particles
  • Heating without chemical change

Correct Answer: Addition of small amounts of strong acid or base

Q46. In the context of pH calculations, the term “weak acid approximation” typically means:

  • Assume percent dissociation is negligible so x << initial concentration
  • Assume full dissociation of the weak acid
  • Ignore the acid dissociation constant Ka
  • Use only pKw to find pH

Correct Answer: Assume percent dissociation is negligible so x << initial concentration

Q47. A drug with pKa 9.5 will be most unionized and most lipophilic at which pH?

  • pH 3
  • pH 9.5
  • pH 12
  • pH 7

Correct Answer: pH 12

Q48. Which statement best describes how pH influences hydrolysis rates of ester drugs?

  • Hydrolysis rates are independent of pH
  • Catalysis can occur by both acid and base mechanisms, often increasing rate at extreme pH
  • Only neutral pH accelerates hydrolysis
  • Hydrolysis stops at pH 7

Correct Answer: Catalysis can occur by both acid and base mechanisms, often increasing rate at extreme pH

Q49. When using Henderson-Hasselbalch for a polyprotic acid, you must:

  • Use the pKa corresponding to the relevant protonation step
  • Always use the lowest pKa only
  • Ignore ionic strength
  • Assume all pKa values are identical

Correct Answer: Use the pKa corresponding to the relevant protonation step

Q50. A practical lesson from Sorensen’s pH concept for pharmaceutical analysis is:

  • pH measurement is purely theoretical with no real application
  • Careful control and measurement of hydrogen ion activity is essential for reproducible pharmaceutical properties
  • One should always report only concentration-based pH without calibration
  • Indicators are sufficient for all regulatory pH determinations

Correct Answer: Careful control and measurement of hydrogen ion activity is essential for reproducible pharmaceutical properties

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