Purity of precipitate in gravimetric analysis MCQs With Answer

Purity of precipitate in gravimetric analysis is a fundamental concept for B.Pharm students performing quantitative determinations in pharmaceutical analysis. Understanding how precipitation mechanisms, co-precipitation (occlusion, adsorption, isomorphous substitution), masking agents, and particle growth affect precipitate purity helps ensure accurate assays. Key procedural controls—pH, temperature, digestion, rate of reagent addition, washing, filtration, drying or ignition, and choice of precipitant—minimize contamination and analytical error. Evaluating and improving purity using reprecipitation, qualitative tests, or chemical treatments is essential for pharmacopeial compliance and reliable quality control. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary objective when maximizing the purity of a precipitate in gravimetric analysis?

• To obtain the largest possible mass of the precipitate
• To ensure the precipitate is amorphous
• To obtain a precipitate free from adhering impurities and foreign ions
• To minimize reagent consumption

Correct Answer: To obtain a precipitate free from adhering impurities and foreign ions

Q2. Which type of co-precipitation involves incorporation of impurity ions into the crystal lattice of the precipitate?

• Occlusion
• Adsorption
• Isomorphic substitution
• Mechanical entrapment

Correct Answer: Isomorphic substitution

Q3. What procedural step reduces occlusion and improves precipitate purity by allowing crystals to grow larger and expel impurities?

• Rapid stirring
• Digestion (aging)
• Immediate filtration
• Excess reagent addition

Correct Answer: Digestion (aging)

Q4. Which washing solvent is most appropriate for removing soluble contaminants from a precipitate without dissolving the precipitate itself?

• Distilled water
• Hot concentrated acid
• Strong organic solvent
• Saturated solution of the precipitate

Correct Answer: Distilled water

Q5. Adsorption as a source of impurity is best described as:

• Entrapment of foreign crystals within growing crystals
• Substitution of ions into the crystal lattice
• Surface binding of ions or colloids to precipitate particles
• Conversion of precipitate to a different chemical form

Correct Answer: Surface binding of ions or colloids to precipitate particles

Q6. Which of the following actions helps minimize adsorption of impurities onto precipitate surfaces?

• Adding excess precipitating reagent
• Washing with a solution containing a common ion or dilute acid/base
• Filtering immediately after precipitation
• Using very fine filter paper

Correct Answer: Washing with a solution containing a common ion or dilute acid/base

Q7. In gravimetric determination of chloride as AgCl, what is a common source of decreased purity of the AgCl precipitate?

• Presence of nitrate ions
• Co-precipitation of silver oxide or silver carbonate
• Using distilled water
• Low temperature during precipitation

Correct Answer: Co-precipitation of silver oxide or silver carbonate

Q8. Which parameter most directly influences the rate of nucleation and thus particle size in precipitation?

• Color of the solution
• Rate of reagent addition
• Molecular weight of analyte
• Type of filtration paper

Correct Answer: Rate of reagent addition

Q9. Which method is commonly used to remove colloidal impurities that adhere to precipitates and reduce purity?

• Electrolysis
• Use of coagulating agents or electrolytes
• Drying at high temperature
• Adding organic dyes

Correct Answer: Use of coagulating agents or electrolytes

Q10. What is the role of a masking agent in gravimetric analysis related to precipitate purity?

• To precipitate the analyte faster
• To form a volatile complex
• To prevent interfering ions from precipitating with the analyte
• To change the color of the precipitate

Correct Answer: To prevent interfering ions from precipitating with the analyte

Q11. Which of the following best explains occlusion as a contamination mechanism?

• Surface adsorption of ions after filtration
• Incorporation of mother liquor or fine particles within crystal aggregates
• Exchange of lattice ions with similar ions
• Formation of a different phase on heating

Correct Answer: Incorporation of mother liquor or fine particles within crystal aggregates

Q12. Why is ignition used after filtration for some gravimetric precipitates?

• To dissolve the precipitate
• To convert the precipitate to a stable, weighable oxide or form
• To disinfect the precipitate
• To increase the precipitate’s solubility

Correct Answer: To convert the precipitate to a stable, weighable oxide or form

Q13. Which practice can cause loss of analyte and reduce apparent purity during washing?

• Using a wash solvent that dissolves some precipitate
• Washing with cold distilled water
• Wetting the precipitate before transfer
• Allowing digestion before washing

Correct Answer: Using a wash solvent that dissolves some precipitate

Q14. Reprecipitation is used to improve purity. What does it involve?

• Heating the precipitate until it melts
• Dissolving the impure precipitate and re-precipitating the analyte under controlled conditions
• Adding more precipitating reagent to the original precipitate
• Mixing the precipitate with inert clay

Correct Answer: Dissolving the impure precipitate and re-precipitating the analyte under controlled conditions

Q15. Which choice best reduces isomorphic substitution during precipitation?

• Using a very high concentration of analyte
• Selecting a precipitant that forms an insoluble compound with dissimilar ionic radius or charge
• Filtering immediately
• Adding organic stabilizers

Correct Answer: Selecting a precipitant that forms an insoluble compound with dissimilar ionic radius or charge

Q16. The use of ashless filter paper is preferred because it:

• Contains minerals that aid filtration
• Leaves negligible residue on ignition
• Is cheaper than regular paper
• Accelerates co-precipitation

Correct Answer: Leaves negligible residue on ignition

Q17. Which analytical sign suggests incomplete precipitation has occurred?

• Clear supernatant after settling
• Persistent turbidity in supernatant
• Crystalline precipitate
• Stable pH

Correct Answer: Persistent turbidity in supernatant

Q18. How does temperature generally affect precipitate purity and crystal growth?

• Higher temperature always increases impurity adsorption
• Elevated temperature can promote crystal growth and improve purity by reducing surface area
• Temperature has no effect on precipitation
• Lower temperature always yields purer precipitates

Correct Answer: Elevated temperature can promote crystal growth and improve purity by reducing surface area

Q19. Which of the following is NOT a method to evaluate precipitate purity?

• Qualitative chemical tests for possible contaminants
• Gravimetric reanalysis after ignition
• X-ray diffraction or spectroscopic analysis
• Ignoring the supernatant composition

Correct Answer: Ignoring the supernatant composition

Q20. What is the effect of rapid addition of precipitating reagent on particle characteristics?

• Favors formation of fewer, larger crystals
• Promotes fewer nucleation sites
• Leads to many small particles and more occlusion/adsorption
• Has no influence on particle formation

Correct Answer: Leads to many small particles and more occlusion/adsorption

Q21. Which precipitate conditioning technique improves filterability and purity?

• Vigorous continuous stirring during filtration
• Allowing the precipitate to digest at elevated temperature
• Adding excessive dispersing agents
• Drying before washing

Correct Answer: Allowing the precipitate to digest at elevated temperature

Q22. A common ion effect is used in washing to:

• Increase solubility of the precipitate
• Suppress dissolution of the precipitate while removing soluble impurities
• Promote co-precipitation
• Convert the precipitate to a volatile form

Correct Answer: Suppress dissolution of the precipitate while removing soluble impurities

Q23. In gravimetry, why are very fine precipitate particles undesirable for purity?

• They settle quickly
• They have greater surface area for adsorption and occlusion of impurities
• They are easier to filter and wash
• They always have perfect crystal structure

Correct Answer: They have greater surface area for adsorption and occlusion of impurities

Q24. Which control is useful when preventing hydrolysis or side reactions that contaminate a precipitate?

• Control of solution pH
• Use of colored indicators only
• Adding inert salts
• Using non-ashless filter paper

Correct Answer: Control of solution pH

Q25. When converting a precipitate to an oxide by ignition, what must be ensured for purity?

• Ignition at the lowest possible temperature
• Complete conversion without volatilizing the analyte
• Addition of organic binders before ignition
• Cooling rapidly in open air

Correct Answer: Complete conversion without volatilizing the analyte

Q26. How does using a precipitant of high purity affect final precipitate purity?

• Has no impact
• Introduces more impurities
• Reduces introduction of foreign ions and improves overall purity
• Always causes precipitation to fail

Correct Answer: Reduces introduction of foreign ions and improves overall purity

Q27. Which analytical practice helps detect co-precipitated metal ion impurities?

• Color observation only
• Dissolving a portion of precipitate and performing specific qualitative tests
• Weighing without further tests
• Adding excess precipitant and assuming purity

Correct Answer: Dissolving a portion of precipitate and performing specific qualitative tests

Q28. During precipitation, colloidal stability can be disrupted by adding electrolytes. This helps because:

• It increases solubility of colloids
• It promotes coagulation and settling of colloidal impurities
• It always forms new soluble complexes
• It decreases ionic strength

Correct Answer: It promotes coagulation and settling of colloidal impurities

Q29. What is a disadvantage of filtering a warm precipitate rather than allowing it to cool and digest?

• Faster filtration
• Higher chance of entraining mother liquor and finer particles, reducing purity
• Improved crystal quality
• Reduced solubility of impurities

Correct Answer: Higher chance of entraining mother liquor and finer particles, reducing purity

Q30. Which practice is essential when performing gravimetric analysis in pharmaceutical quality control to ensure precipitate purity and reliable results?

• Ignoring method validation
• Following validated procedures, controlling variables, and verifying precipitate purity by tests
• Using random reagents and conditions
• Relying solely on visual inspection of precipitate

Correct Answer: Following validated procedures, controlling variables, and verifying precipitate purity by tests

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