Principle of precipitation titrations MCQs With Answer

Principle of precipitation titrations MCQs With Answer

Precipitation titrations are titrimetric methods where the analytical measurement is based on the formation of an insoluble solid. This topic covers fundamentals such as solubility product (Ksp), stoichiometry of precipitation reactions, common-ion and ionic-strength effects, endpoint detection using specific indicators, and classic argentometric methods (Mohr, Volhard, Fajans). It also examines factors affecting accuracy—digestion, co-precipitation, masking agents, and back-titration—important for pharmaceutical assay and quality control. Understanding these principles helps B.Pharm students interpret titration curves, calculate concentrations, and select suitable indicators and conditions for reliable precipitative analyses. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary analytical principle behind precipitation titrations?

• Formation of an insoluble precipitate according to stoichiometry
• Neutralization of acids and bases
• Oxidation-reduction electron transfer
• Complex formation without precipitation

Correct Answer: Formation of an insoluble precipitate according to stoichiometry

Q2. In an argentometric titration for chloride, which titrant is commonly used?

• Silver nitrate
• Potassium permanganate
• Standardized hydrochloric acid
• Ammonium thiocyanate

Correct Answer: Silver nitrate

Q3. The solubility product constant (Ksp) is best described as:

• The equilibrium constant for dissolution of a sparingly soluble salt
• The rate constant for precipitation reactions
• The pH at which precipitation begins
• A measure of ionic strength

Correct Answer: The equilibrium constant for dissolution of a sparingly soluble salt

Q4. Which of the following argentometric methods uses adsorption indicators to detect the endpoint?

• Fajans method
• Mohr method
• Volhard method
• Titrimetric neutralization method

Correct Answer: Fajans method

Q5. The Mohr method employs which indicator to detect the endpoint for chloride titration?

• Potassium chromate
• Fluorescein
• Phenolphthalein
• Diphenylcarbazide

Correct Answer: Potassium chromate

Q6. In Volhard titration, the endpoint is detected by which type of indicator?

• Complexation indicator (e.g., ferric ion as indicator for thiocyanate)
• Fluorescent adsorption indicator
• pH indicator
• Redox potential indicator

Correct Answer: Complexation indicator (e.g., ferric ion as indicator for thiocyanate)

Q7. Which factor most directly shifts the position of a precipitation equilibrium according to Le Chatelier’s principle?

• Addition of a common ion
• Changing the color of the indicator
• Using a stronger acid
• Increasing the stirring speed only

Correct Answer: Addition of a common ion

Q8. Solubility of a salt is generally decreased by:

• Adding a common ion
• Increasing temperature for all salts
• Decreasing ionic strength
• Adding a complexing ligand

Correct Answer: Adding a common ion

Q9. Which precipitation titration approach is preferred when analyte forms a very fine precipitate that causes turbidity and slow settling?

• Use of digestion and controlled warming to improve crystal size
• Perform titration at very low temperature
• Avoid titration and use spectrophotometry only
• Increase mixing speed to keep precipitate suspended

Correct Answer: Use of digestion and controlled warming to improve crystal size

Q10. For a salt MX2 with Ksp = [M2+][X-]^2, what happens to [X-] at equilibrium if [M2+] is doubled?

• [X-] decreases to maintain Ksp, specifically divided by square root of 2
• [X-] doubles
• [X-] remains unchanged
• [X-] quadruples

Correct Answer: [X-] decreases to maintain Ksp, specifically divided by square root of 2

Q11. Which problem is most associated with co-precipitation in precipitation titrations?

• Contamination of the precipitate causing systematic error
• Faster endpoint detection
• Complete dissolution of precipitate
• Colorless endpoint making detection easy

Correct Answer: Contamination of the precipitate causing systematic error

Q12. Masking agents are used in precipitation titrations to:

• Prevent interference by forming soluble complexes with interfering ions
• Enhance precipitation by reducing solubility
• Change the color of the indicator
• Increase ionic strength only

Correct Answer: Prevent interference by forming soluble complexes with interfering ions

Q13. In determining chloride by Mohr method, which condition must be strictly avoided?

• Acidic conditions that convert chromate to dichromate
• Presence of excess silver nitrate
• Working at room temperature
• Using standardized titrant

Correct Answer: Acidic conditions that convert chromate to dichromate

Q14. Which of the following best describes the endpoint in a precipitation titration?

• The point at which the last stoichiometric amount of analyte has precipitated and indicator shows a change
• The point where pH is neutral
• The point where titrant concentration is maximum
• Any point before equivalence for practical reasons

Correct Answer: The point at which the last stoichiometric amount of analyte has precipitated and indicator shows a change

Q15. In a back-titration for a poorly soluble analyte, the primary advantage is:

• Improved accuracy by reacting excess reagent and titrating back
• Elimination of need for standard solutions
• Shorter titration time always
• No need for endpoint detection

Correct Answer: Improved accuracy by reacting excess reagent and titrating back

Q16. Which of the following ions commonly interferes in chloride determination by argentometry and must be removed or masked?

• Bromide
• Sodium
• Potassium
• Calcium at trace levels

Correct Answer: Bromide

Q17. The term “digestion” in precipitation titration refers to:

• Allowing precipitate to age at elevated temperature to form larger crystals
• Acidifying the solution to dissolve precipitate
• Filtering immediately without washing
• Adding chelating agents to prevent precipitation

Correct Answer: Allowing precipitate to age at elevated temperature to form larger crystals

Q18. In a titration where AgCl is formed, which of the following will shift the equivalence point potential if using a potentiometric endpoint?

• Change in chloride concentration near equivalence affecting silver activity
• Using a glass pH electrode instead of a silver electrode
• Temperature change only if pH is maintained
• Addition of an inert salt with no ionic strength change

Correct Answer: Change in chloride concentration near equivalence affecting silver activity

Q19. In Fajans titration, the indicator adsorption changes sign when:

• Surface charge of the precipitate switches after the equivalence point
• The solution pH reaches 7
• The precipitate completely dissolves
• Temperature drops below 0 °C

Correct Answer: Surface charge of the precipitate switches after the equivalence point

Q20. Which calculation is necessary to predict whether precipitation will occur when two solutions are mixed?

• Compare ionic product (Q) with Ksp
• Calculate redox potential of the mixture
• Determine solution viscosity
• Measure total dissolved solids only

Correct Answer: Compare ionic product (Q) with Ksp

Q21. When performing a titration that forms a colloidal precipitate, the most likely source of error is:

• Incomplete settling leading to loss during filtration and poor endpoint clarity
• Overestimation of titrant concentration only
• Excess formation of large crystals improving accuracy
• Faster equilibration with no practical issues

Correct Answer: Incomplete settling leading to loss during filtration and poor endpoint clarity

Q22. For sulfate determination by precipitation, which precipitating ion is commonly used?

• Barium ion to form barium sulfate
• Silver ion to form silver sulfate
• Lead ion to form lead sulfate
• Calcium ion to form calcium sulfate

Correct Answer: Barium ion to form barium sulfate

Q23. In a precipitation titration, increasing ionic strength typically:

• Reduces activity coefficients and can increase apparent solubility
• Always decreases solubility of salts
• Has no effect on solubility or activity
• Eliminates the need for indicators

Correct Answer: Reduces activity coefficients and can increase apparent solubility

Q24. Which statement about Mohr and Fajans methods is correct?

• Mohr uses chromate as indicator forming a red silver chromate; Fajans uses adsorption of dyes
• Both methods use the same indicator under identical conditions
• Fajans requires strongly acidic media like Mohr
• Mohr is based on potentiometric detection only

Correct Answer: Mohr uses chromate as indicator forming a red silver chromate; Fajans uses adsorption of dyes

Q25. During analysis, formation of complex ions (e.g., with ammonia) affects precipitation titrations by:

• Increasing solubility of the metal ion and preventing precipitation
• Always decreasing solubility and promoting precipitation
• Having no effect on titration results
• Changing color of the precipitate only

Correct Answer: Increasing solubility of the metal ion and preventing precipitation

Q26. A titration of chloride requires pH control because:

• Indicator stability and chromate behavior can change with pH
• Chloride ion is protonated at high pH
• Silver nitrate decomposes in neutral media
• pH does not influence precipitation titrations

Correct Answer: Indicator stability and chromate behavior can change with pH

Q27. In the context of pharmaceutical analysis, why are precipitation titrations still relevant?

• They provide accurate, low-cost assays for halides, sulfates and metal contaminants
• They have replaced all chromatographic techniques
• They require no sample preparation at all
• They are only of historical interest and not used practically

Correct Answer: They provide accurate, low-cost assays for halides, sulfates and metal contaminants

Q28. Which technique helps to minimize occlusion errors in precipitate-based gravimetric steps?

• Washing precipitate with suitable wash solutions and digestion
• Filtering through large pore paper immediately without washing
• Drying at very high temperature instantly
• Adding excess titrant to ensure occlusion

Correct Answer: Washing precipitate with suitable wash solutions and digestion

Q29. The ionic product Q for a salt AgCl equals [Ag+][Cl-]. If Q < Ksp, what is expected?

• No precipitation will occur; solution remains unsaturated
• Immediate and complete precipitation
• Solution must be acidic to precipitate
• Spontaneous dissolution of container

Correct Answer: No precipitation will occur; solution remains unsaturated

Q30. Which procedural modification is useful when titrating samples containing organic solvents that affect precipitation?

• Use a solvent-compatible indicator and adjust ionic strength or remove solvent by extraction
• Ignore solvent effects and proceed normally
• Always heat the mixture to boiling to force precipitation
• Use pure organic solvent as titrant

Correct Answer: Use a solvent-compatible indicator and adjust ionic strength or remove solvent by extraction

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