Introduction
Mohr’s method of precipitation titration is a classical argentometric titration technique used to determine chloride (and sometimes bromide) concentrations using silver nitrate and potassium chromate as an indicator. Widely taught in B.Pharm courses, it emphasizes stoichiometry, solubility product (Ksp), endpoint recognition, and practical lab skills. Key concepts include formation of AgCl precipitate, appearance of red-brown silver chromate at the endpoint, effects of pH, ionic strength, interfering ions, and selection of appropriate samples and indicators. Understanding sample preparation, procedural steps, and common errors is essential for accurate results. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the primary analyte determined by Mohr’s method?
- Chloride ions (Cl-)
- Sulfate ions (SO4 2-)
- Nitrate ions (NO3-)
- Potassium ions (K+)
Correct Answer: Chloride ions (Cl-)
Q2. Which reagent is used as the titrant in Mohr’s method?
- Potassium chromate
- Silver nitrate
- Sodium chloride
- Ammonium thiocyanate
Correct Answer: Silver nitrate
Q3. What indicator is typically used in Mohr’s method?
- Potassium chromate
- Phenolphthalein
- Methyl orange
- Eriochrome black T
Correct Answer: Potassium chromate
Q4. The visible endpoint in Mohr’s method is indicated by formation of which compound?
- Red-brown silver chromate (Ag2CrO4)
- White silver chloride (AgCl)
- Yellow silver iodide (AgI)
- Blue silver ferrocyanide
Correct Answer: Red-brown silver chromate (Ag2CrO4)
Q5. Which stoichiometric reaction represents the titration before the endpoint?
- Ag+ + CrO4 2- → Ag2CrO4
- Ag+ + Cl- → AgCl (s)
- Ag+ + NO3- → AgNO3
- 2Ag+ + SO4 2- → Ag2SO4
Correct Answer: Ag+ + Cl- → AgCl (s)
Q6. Why must the solution pH be kept near neutral in Mohr’s method?
- Acidic conditions dissolve AgCl and prevent precipitation
- Alkaline conditions convert chromate to dichromate affecting endpoint
- pH has no effect on Mohr’s titration
- Extreme pH increases Ag+ concentration artificially
Correct Answer: Alkaline conditions convert chromate to dichromate affecting endpoint
Q7. Which of the following ions interferes strongly by forming a precipitate with Ag+ before chloride is fully consumed?
- Sodium (Na+)
- Bromide (Br-)
- Sulfate (SO4 2-)
- Ammonium (NH4+)
Correct Answer: Bromide (Br-)
Q8. What is the role of potassium chromate in Mohr’s method?
- Acts as a reducing agent
- Acts as a secondary reactant to precipitate chloride
- Serves as an external indicator forming colored precipitate with excess Ag+
- Complexes with chloride to increase solubility
Correct Answer: Serves as an external indicator forming colored precipitate with excess Ag+
Q9. Which equation defines the solubility product for silver chloride relevant to Mohr’s method?
- Ksp = [Ag+][NO3-]
- Ksp = [Ag+][Cl-]
- Ksp = [Ag+]^2[Cl-]^2
- Ksp = [Ag+][CrO4 2-]
Correct Answer: Ksp = [Ag+][Cl-]
Q10. Why is Mohr’s method unsuitable for strongly acidic solutions?
- Chromate indicator reacts with H+ forming dichromate, masking endpoint
- AgCl solubility increases significantly with acid
- Silver nitrate decomposes in acid
- Chloride concentration decreases in acid
Correct Answer: Chromate indicator reacts with H+ forming dichromate, masking endpoint
Q11. How is the endpoint visually detected in a typical Mohr titration?
- Sudden turbidity due to AgCl
- Appearance of persistent red-brown color from Ag2CrO4
- Solution turns clear and colorless
- Formation of white foam
Correct Answer: Appearance of persistent red-brown color from Ag2CrO4
Q12. Which procedural step improves accuracy in Mohr’s method when titrating colored samples?
- Use a lower concentration of titrant
- Add excess chromate indicator at start
- Perform blank titration and use back titration if necessary
- Raise pH to strongly alkaline
Correct Answer: Perform blank titration and use back titration if necessary
Q13. What happens if too much chromate indicator is added before titration?
- No effect on endpoint
- False early red-brown endpoint due to excess chromate
- Increases solubility of AgCl
- Precipitates silver chromate immediately
Correct Answer: False early red-brown endpoint due to excess chromate
Q14. Compared to Fajans’ method, Mohr’s method uses which of the following?
- An adsorption indicator (dye)
- A precipitation indicator forming a colored precipitate
- No indicator at all
- Complexometric titrant
Correct Answer: A precipitation indicator forming a colored precipitate
Q15. Which parameter of the titrant must be known accurately for quantitative Mohr titration?
- Density of silver nitrate solution
- Exact molarity (standardization) of AgNO3
- Color of AgNO3
- pH of AgNO3 solution
Correct Answer: Exact molarity (standardization) of AgNO3
Q16. Which sample preparation step is important before performing Mohr’s titration on an ionic pharmaceutical formulation?
- Remove excess chloride by heating to dryness
- Dissolve sample and filter to remove insolubles; dilute to known volume
- Add silver nitrate before dissolving
- Acidify strongly with HCl
Correct Answer: Dissolve sample and filter to remove insolubles; dilute to known volume
Q17. Which of these will lower the solubility of AgCl and thus affect the titration curve?
- Increasing temperature drastically
- Adding a common ion like Cl-
- Lowering ionic strength
- Complexing Ag+ with ammonia
Correct Answer: Adding a common ion like Cl-
Q18. For accurate detection, the concentration of chromate indicator is usually:
- Much higher than chloride concentration
- Approximately equal to chloride molarity
- Very small, catalytic amounts
- Stoichiometric with silver nitrate
Correct Answer: Very small, catalytic amounts
Q19. When titrating in presence of bromide, what adjustment can make Mohr’s method selective for chloride?
- Lowering pH to convert bromide to bromine
- Precipitating bromide first by selective masking or removal
- Using phenolphthalein indicator instead
- Increasing AgNO3 concentration only
Correct Answer: Precipitating bromide first by selective masking or removal
Q20. The formation of Ag2CrO4 as endpoint occurs because silver reacts preferentially with which species once chloride is exhausted?
- Chromate (CrO4 2-)
- Nitrate (NO3-)
- Sulfate (SO4 2-)
- Water molecules
Correct Answer: Chromate (CrO4 2-)
Q21. Which mathematical calculation is essential after titration to find chloride concentration?
- Use Henderson–Hasselbalch equation
- Moles of Ag+ consumed = moles of Cl- present; convert to concentration
- Use Beer–Lambert law
- Calculate ionic strength only
Correct Answer: Moles of Ag+ consumed = moles of Cl- present; convert to concentration
Q22. Which of the following will form a complex with Ag+ and prevent precipitation with chloride?
- Sodium chloride
- Ammonia (NH3) forming [Ag(NH3)2]+
- Potassium chromate
- Hydrochloric acid
Correct Answer: Ammonia (NH3) forming [Ag(NH3)2]+
Q23. If end-point color is faint and unstable, a recommended remedy is:
- Increase titrant concentration drastically
- Add a small amount of ethanol to change solubility
- Standardize titrant again and repeat titration using smaller aliquot
- Ignore and record equivalence earlier
Correct Answer: Standardize titrant again and repeat titration using smaller aliquot
Q24. Which safety precaution is specifically relevant to Mohr’s titration due to reagents used?
- Avoid inhaling silver nitrate and chromate solutions due to toxicity
- Always heat chromate solutions to boil
- Wear ear protection
- Store AgNO3 under sunlight only
Correct Answer: Avoid inhaling silver nitrate and chromate solutions due to toxicity
Q25. A typical concentration range of chloride suitable for Mohr titration in volumetric analysis is:
- Trace ppb levels only
- Millimolar to percent levels (mg to g per liter)
- Purely molar (>1 M) solutions only
- Any concentration without dilution
Correct Answer: Millimolar to percent levels (mg to g per liter)
Q26. Which alternative argentometric method is preferred when samples are acidic and Mohr cannot be used?
- Fajans method
- Volhard method (back titration)
- Complexometric EDTA titration
- Gravimetric silver analysis
Correct Answer: Volhard method (back titration)
Q27. Which physical observation indicates that precipitation of AgCl is complete before endpoint?
- Solution becomes deeply colored red
- Cloudiness due to fine white AgCl particles stabilizes
- Gas evolution occurs
- Solution heats up significantly
Correct Answer: Cloudiness due to fine white AgCl particles stabilizes
Q28. What is a primary limitation of Mohr’s method in pharmaceutical analysis?
- Cannot detect chloride at all
- Interference from colored or oxidizing excipients and bromide/iodide
- Requires extremely high temperatures
- Works only for organic chlorides
Correct Answer: Interference from colored or oxidizing excipients and bromide/iodide
Q29. During titration, why should strong complexing agents be absent from samples?
- They catalyze AgCl precipitation
- They bind Ag+, preventing complete precipitation of chloride and causing error
- They convert chromate to chromic acid
- They enhance endpoint color intensity
Correct Answer: They bind Ag+, preventing complete precipitation of chloride and causing error
Q30. For best precision in Mohr’s titration, which practice is recommended?
- Use unstandardized reagents to save time
- Perform replicate titrations and average results; standardize AgNO3 frequently
- Add large excess of chromate indicator to sharpen color
- Conduct titration at strongly acidic pH
Correct Answer: Perform replicate titrations and average results; standardize AgNO3 frequently
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