Fajans method and estimation of sodium chloride MCQs With Answer

Fajans method is an adsorption indicator technique widely used in pharmaceutical analysis for the accurate estimation of sodium chloride. This argentometric titration relies on the adsorption of a colored indicator on the surface of a precipitate (AgCl), producing a sharp visual endpoint. Key concepts include adsorption indicators (e.g., dichlorofluorescein), precipitate formation, colloidal surface charge, equivalence vs endpoint, interferences, and calculation of chloride content to determine sodium chloride. Understanding procedure steps, reagent standardization, and common errors is essential for B. Pharm students performing salt assays in quality control. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the fundamental principle of the Fajans method in estimating sodium chloride?

• Complexation of chloride with a colored ligand
• Precipitation followed by adsorption of an indicator on the precipitate
• Redox reaction between chloride and an oxidizing agent
• Volumetric displacement of sodium ions

Correct Answer: Precipitation followed by adsorption of an indicator on the precipitate

Q2. Which of the following indicators is most commonly used in the Fajans method for chloride estimation?

• Methyl orange
• Phenolphthalein
• Dichlorofluorescein
• Potassium chromate

Correct Answer: Dichlorofluorescein

Q3. In the Fajans method, which silver compound precipitates when AgNO3 is added to a sodium chloride solution?

• Ag2O
• AgCl
• Ag2CO3
• AgBr

Correct Answer: AgCl

Q4. What causes the color change at the endpoint in Fajans titration?

• Formation of a soluble silver–indicator complex
• Indicator adsorption on the charged surface of AgCl after equivalence
• Change in pH due to proton release
• Oxidation of the indicator by silver ions

Correct Answer: Indicator adsorption on the charged surface of AgCl after equivalence

Q5. How does the surface charge of AgCl particles change during the Fajans titration just before and after the equivalence point?

• Remains positive before and negative after equivalence
• Negative before equivalence and becomes positive after equivalence
• Always neutral
• Positive both before and after equivalence

Correct Answer: Negative before equivalence and becomes positive after equivalence

Q6. Why is the Fajans method preferred over Mohr method in some pharmaceutical determinations?

• It does not require careful pH control and is suitable for colored solutions
• It uses cheaper reagents exclusively
• It is unaffected by bromide and iodide
• It is a redox titration with greater precision

Correct Answer: It does not require careful pH control and is suitable for colored solutions

Q7. Which interference is most critical for the Fajans method when estimating chloride?

• Presence of sodium ions
• Bromide or iodide ions that also precipitate with Ag+
• Excess nitrate in solution
• Low ionic strength

Correct Answer: Bromide or iodide ions that also precipitate with Ag+

Q8. For a typical Fajans titration, what is the role of adding a small amount of dye indicator such as dichlorofluorescein?

• To complex with chloride to form a colored complex
• To discolor the solution at the start for better visibility
• To adsorb onto AgCl surface and signal the endpoint by color reversal
• To precipitate silver ions before titration

Correct Answer: To adsorb onto AgCl surface and signal the endpoint by color reversal

Q9. Which statement about the endpoint and equivalence point in Fajans titration is correct?

• Endpoint and equivalence point always coincide exactly
• Endpoint occurs slightly after the equivalence point due to indicator adsorption
• Endpoint occurs before the equivalence point because of excess analyte
• There is no equivalence point in precipitation titrations

Correct Answer: Endpoint occurs slightly after the equivalence point due to indicator adsorption

Q10. During Fajans titration of NaCl, which factor increases the sharpness of the endpoint?

• High concentration of sodium chloride sample
• Using high ionic strength and fast stirring to form fine precipitate
• Very slow addition of concentrated silver nitrate without stirring
• Performing titration at elevated temperatures only

Correct Answer: Using high ionic strength and fast stirring to form fine precipitate

Q11. How do you calculate sodium chloride content from chloride titration data obtained by Fajans method?

• Use molarity of AgNO3 and volume at endpoint to get moles Cl-, then convert moles Cl- to NaCl by 1:1 stoichiometry and molecular weight
• Multiply volume of AgNO3 by 58.44 directly
• Use titrant volume and assume 2:1 stoichiometry of Ag+ to Cl-
• Estimate sodium chloride by difference in solution density before and after titration

Correct Answer: Use molarity of AgNO3 and volume at endpoint to get moles Cl-, then convert moles Cl- to NaCl by 1:1 stoichiometry and molecular weight

Q12. In a Fajans titration, if 25.00 mL of 0.1000 M AgNO3 was required to reach the endpoint for a sample, how many millimoles of chloride were present?

• 0.250 mmol
• 2.500 mmol
• 25.00 mmol
• 0.025 mmol

Correct Answer: 2.500 mmol

Q13. Which precaution is important when preparing the AgNO3 titrant for Fajans titration?

• Prepare titrant in non-opaque glass to allow light exposure
• Standardize titrant against a primary standard like sodium chloride or potassium chloride
• Use tap water to prepare titrant to maintain ionic strength
• Keep titrant acidic with strong acid to prevent precipitation

Correct Answer: Standardize titrant against a primary standard like sodium chloride or potassium chloride

Q14. Which of the following best describes the adsorption indicator behavior in Fajans method?

• Indicator forms a soluble colored complex with chloride in bulk solution
• Indicator adsorbs to positively charged AgCl surface after equivalence and shows indicator color
• Indicator changes color due to pH shift caused by silver ions
• Indicator polymerizes at the equivalence point creating turbidity

Correct Answer: Indicator adsorbs to positively charged AgCl surface after equivalence and shows indicator color

Q15. Which solvent condition can negatively affect the Fajans titration by dissolving AgCl precipitate?

• Use of concentrated NaCl solution only
• Presence of ammonia, which forms soluble [Ag(NH3)2]+
• Presence of inert salts like KNO3
• Low temperature

Correct Answer: Presence of ammonia, which forms soluble [Ag(NH3)2]+

Q16. For pharmaceutical samples containing color or turbidity, why might Fajans method still be preferable to Mohr?

• Fajans endpoint is based on adsorption color change which can be observed on precipitate surface despite colored solution
• Fajans does not require a visual endpoint at all
• Fajans uses a fluorescent detector rather than human observation
• Mohr method is only for organic analytes

Correct Answer: Fajans endpoint is based on adsorption color change which can be observed on precipitate surface despite colored solution

Q17. Which experimental step helps to prevent colloidal aggregation and ensures reproducible Fajans endpoints?

• Adding titrant in large aliquots quickly
• Maintaining constant and vigorous stirring during titration
• Allowing precipitate to settle completely before adding more titrant
• Heating to boiling point during titration

Correct Answer: Maintaining constant and vigorous stirring during titration

Q18. In the context of Fajans titration, what is the role of ionic strength in the titration medium?

• Ionic strength controls solubility and influences double-layer thickness affecting indicator adsorption
• Ionic strength directly reacts with the indicator causing color change
• Ionic strength is irrelevant to precipitation titrations
• Ionic strength replaces chloride in the precipitate

Correct Answer: Ionic strength controls solubility and influences double-layer thickness affecting indicator adsorption

Q19. Which property of the precipitate surface is central to the adsorption indicator mechanism in Fajans method?

• Surface pH only
• Surface charge and zeta potential
• Crystal lattice energy exclusively
• Magnetic susceptibility of the precipitate

Correct Answer: Surface charge and zeta potential

Q20. If bromide is present in a sample, how can it affect a Fajans titration for chloride?

• Bromide will not react with Ag+ and has no effect
• Bromide will precipitate earlier or co-precipitate with Ag+, causing high bias in chloride estimation
• Bromide converts the indicator to an inactive form
• Bromide increases the volatility of chloride

Correct Answer: Bromide will precipitate earlier or co-precipitate with Ag+, causing high bias in chloride estimation

Q21. Which titration parameter is most commonly standardized before performing Fajans titration for NaCl estimation?

• Indicator concentration only
• Concentration (normality/molarity) of AgNO3 titrant
• Temperature of the laboratory
• pH of distilled water used

Correct Answer: Concentration (normality/molarity) of AgNO3 titrant

Q22. Which observation indicates that the endpoint has been reached in a Fajans titration using dichlorofluorescein?

• Solution becomes colorless
• Fine pink or red tinge appears on the surface of the precipitate
• Formation of a green gas
• Precipitate dissolves completely

Correct Answer: Fine pink or red tinge appears on the surface of the precipitate

Q23. When converting chloride assay results to percent sodium chloride in a pharmaceutical sample, which molecular weight is used for NaCl?

• 22.99 g/mol
• 35.45 g/mol
• 58.44 g/mol
• 107.87 g/mol

Correct Answer: 58.44 g/mol

Q24. What is a primary standard commonly used to standardize AgNO3 titrant for chloride estimation?

• Potassium dichromate
• Potassium chloride (dry, pure)
• Acetic acid
• Sodium hydroxide

Correct Answer: Potassium chloride (dry, pure)

Q25. During Fajans titration, why should strong complexing agents like cyanide be avoided in sample or reagents?

• They increase the precipitation rate of AgCl
• They form soluble complexes with Ag+ and prevent precipitation, causing endpoint errors
• They convert chloride into chloride radicals
• They cause excessive foaming

Correct Answer: They form soluble complexes with Ag+ and prevent precipitation, causing endpoint errors

Q26. Which titration variant uses an internal indicator (silver electrode) rather than adsorption indicators and is often used to avoid color interference?

• Volhard method
• Mohr method
• Potentiometric argentometric titration
• Complexometric titration

Correct Answer: Potentiometric argentometric titration

Q27. In routine assay of NaCl by Fajans method, a student obtains a consistently late endpoint. Which procedural error is most likely?

• Using too little indicator
• Adding titrant too slowly with vigorous stirring
• Using excessively large drops of titrant leading to overshoot
• High turbidity preventing adsorption

Correct Answer: Using excessively large drops of titrant leading to overshoot

Q28. Which temperature effect is relevant for Fajans titration of NaCl?

• Higher temperature always sharpens the endpoint by stabilizing the precipitate
• Temperature affects solubility of AgCl and can change precipitation behavior and endpoint sharpness
• Temperature has no effect on precipitation titrations
• Lower temperature converts chloride to chloride hydrate preventing titration

Correct Answer: Temperature affects solubility of AgCl and can change precipitation behavior and endpoint sharpness

Q29. Why is it important to wash the precipitate when isolating AgCl for gravimetric confirmation after a Fajans titration?

• To remove physically adsorbed indicator and soluble salts that would bias gravimetric mass
• To dissolve the AgCl back into solution for analysis
• To increase the mass by adding water content intentionally
• To neutralize the charge on the precipitate surface

Correct Answer: To remove physically adsorbed indicator and soluble salts that would bias gravimetric mass

Q30. Which statement best summarizes the suitability of the Fajans method for quality control in pharmaceutical formulations?

• Fajans method is obsolete and never used in pharmaceutical QC
• Fajans method is useful for routine NaCl estimation when controls for interferences, indicator choice, and technique are applied
• Fajans method can quantify all halides without sample pretreatment
• Fajans method always yields exact gravimetric results without error

Correct Answer: Fajans method is useful for routine NaCl estimation when controls for interferences, indicator choice, and technique are applied

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