Introduction
Gravimetric analysis is a classical quantitative technique in analytical chemistry widely used in B. Pharm courses to determine the amount of an analyte by measuring the mass of a pure, stable compound. Key principles and steps in gravimetric analysis include precipitation, digestion, filtration, washing, drying or ignition, and accurate weighing. Mastery of precipitant selection, control of pH and temperature, prevention of co-precipitation and adsorption, and calculations using the gravimetric factor ensures precision and accuracy in pharmaceutical assays. This concise review emphasizes practical procedure, typical precipitates, sources of error and corrective strategies. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the basic principle of gravimetric analysis?
- Quantitative determination by measuring mass of an analyte or its derivative
- Quantitative determination by measuring solution color intensity
- Quantitative determination by measuring electrical current
- Qualitative identification of ions by flame color
Correct Answer: Quantitative determination by measuring mass of an analyte or its derivative
Q2. Which step most directly provides the quantitative result in precipitation gravimetry?
- Filtration and rinsing of the filtrate
- Precipitation and weighing of an insoluble compound
- Calibration of the balance
- Use of an internal standard
Correct Answer: Precipitation and weighing of an insoluble compound
Q3. Which are the three major types of gravimetric methods?
- Titrimetric, colorimetric and potentiometric
- Chromatographic, spectrophotometric and electrochemical
- Precipitation, volatilization and electrogravimetry
- Filtration, centrifugation and decantation
Correct Answer: Precipitation, volatilization and electrogravimetry
Q4. Which procedural step is primarily used to remove soluble impurities from a precipitate?
- Digestion
- Washing
- Ignition
- Seeding
Correct Answer: Washing
Q5. What is the purpose of digestion (aging) of a freshly formed precipitate?
- To increase particle size and improve purity and filterability
- To dissolve the precipitate for re-precipitation
- To neutralize the sample solution
- To oxidize the analyte
Correct Answer: To increase particle size and improve purity and filterability
Q6. Which reagent is most commonly used to precipitate chloride as a gravimetric solid?
- BaCl2 solution
- Silver nitrate solution
- Sodium carbonate solution
- Lead acetate solution
Correct Answer: Silver nitrate solution
Q7. How is the gravimetric factor defined?
- The ratio of the molar mass of the precipitate to the analyte molar mass
- The conversion factor to calculate analyte mass from the mass of the weighed precipitate
- The mass of precipitate formed per unit volume of solution
- The fraction of precipitate lost during washing
Correct Answer: The conversion factor to calculate analyte mass from the mass of the weighed precipitate
Q8. What is co-precipitation and why does it reduce accuracy?
- Formation of colloids that improve filtration
- Incorporation of impurities into the precipitate, causing positive error
- Loss of precipitate due to incomplete digestion
- Conversion of precipitate to volatile species
Correct Answer: Incorporation of impurities into the precipitate, causing positive error
Q9. Which approach is commonly used to prevent interfering ions from co-precipitating?
- Seeding with inert crystals
- Rapid cooling of the solution
- Use of masking agents that complex interfering ions
- Increasing ionic strength indiscriminately
Correct Answer: Use of masking agents that complex interfering ions
Q10. Why is ignition performed on many precipitates before final weighing?
- To dissolve the precipitate for spectrophotometric assay
- To volatilize the analyte completely
- To convert the precipitate to a stable, known composition (often oxide) for accurate weighing
- To change particle size for better filtration
Correct Answer: To convert the precipitate to a stable, known composition (often oxide) for accurate weighing
Q11. Which filtration device is preferred for very fine, slowly settling precipitates?
- Ordinary filter paper
- Sintered glass crucible (porous disc)
- Funnel without filter
- Open beaker decantation
Correct Answer: Sintered glass crucible (porous disc)
Q12. How do you obtain the analyte mass from the mass of the isolated precipitate?
- By subtracting the mass of filter paper
- By multiplying the precipitate mass by the gravimetric factor
- By dividing by the molar mass of water
- By adding the mass of the reagent used
Correct Answer: By multiplying the precipitate mass by the gravimetric factor
Q13. Which is an example of volatilization gravimetry?
- Precipitation of BaSO4 for sulfate determination
- Sublimation and weighing of elemental iodine
- Electrolytic deposition of copper
- Gravimetric determination using AgCl
Correct Answer: Sublimation and weighing of elemental iodine
Q14. What is the principle of electrogravimetry?
- Measurement of precipitate turbidity
- Electrochemical deposition of the analyte onto an electrode and weighing the deposit
- Volatilization of analyte as a gas
- Complexation followed by precipitation
Correct Answer: Electrochemical deposition of the analyte onto an electrode and weighing the deposit
Q15. Why is pH control critical during many gravimetric precipitations?
- pH affects the color of the precipitate only
- Precipitate solubility and the chemical form of the analyte depend on pH
- pH changes the balance calibration
- pH is irrelevant in gravimetric methods
Correct Answer: Precipitate solubility and the chemical form of the analyte depend on pH
Q16. What characterizes homogeneous precipitation?
- Instant addition of a strong precipitant to produce rapid nucleation
- Slow in situ generation of the precipitating agent to yield uniform crystals
- Filtration before precipitation is complete
- Use of mechanical stirring only
Correct Answer: Slow in situ generation of the precipitating agent to yield uniform crystals
Q17. Which precipitate is typically used for gravimetric determination of sulfate?
- Silver chloride (AgCl)
- Barium sulfate (BaSO4)
Correct Answer: Barium sulfate (BaSO4)
Q18. Why are precipitates usually washed with small volumes of cold distilled water or cold dilute reagent?
- To speed up dissolution of the precipitate
- To remove soluble impurities while minimizing dissolution of the precipitate
- To oxidize organic impurities
- To neutralize the precipitate
Correct Answer: To remove soluble impurities while minimizing dissolution of the precipitate
Q19. Which is a common major source of systematic error in gravimetric analysis?
- Co-precipitation of impurities with the primary precipitate
- Use of high-precision balances
- Excessive filtration speed
- Ignoring color changes
Correct Answer: Co-precipitation of impurities with the primary precipitate
Q20. How does formation of colloidal precipitates affect gravimetric work?
- Colloids improve settling and ease filtration
- Colloidal particles are so small and stable that filtration becomes difficult, reducing recovery
- Colloids are easily removed by washing with hot solvent
- Colloids enhance the purity of precipitate
Correct Answer: Colloidal particles are so small and stable that filtration becomes difficult, reducing recovery
Q21. What is the role of seeding during precipitation?
- To dissolve the precipitate intentionally
- To provide nucleation sites promoting controlled crystal growth and larger crystals
- To neutralize the solution pH
- To increase co-precipitation
Correct Answer: To provide nucleation sites promoting controlled crystal growth and larger crystals
Q22. Which property is essential for a precipitate to be suitable for gravimetric determination?
- High solubility in water
- Formation of a stoichiometric, low-solubility precipitate with known composition
- Strong color for visual detection
- Formation only at very high temperatures
Correct Answer: Formation of a stoichiometric, low-solubility precipitate with known composition
Q23. How can adsorption of impurities on a precipitate surface be minimized?
- Washing with dilute solution of the precipitating reagent to displace adsorbed ions
- Using boiling concentrated acid for washing
- Drying without washing
- Adding large excess of unrelated salts
Correct Answer: Washing with dilute solution of the precipitating reagent to displace adsorbed ions
Q24. Which formula correctly expresses the gravimetric factor (GF)?
- GF = mass of precipitate / volume of original solution
- GF = (molar mass of analyte × stoichiometric coefficient in precipitate) / molar mass of precipitate
- GF = mass of filter paper × sample mass
- GF = molar mass of precipitate × solubility product
Correct Answer: GF = (molar mass of analyte × stoichiometric coefficient in precipitate) / molar mass of precipitate
Q25. If the analyte concentration is too low to form an easily filterable precipitate, what strategy can be employed?
- Ignore the sample and report zero
- Add a carrier to facilitate precipitation by co-precipitation
- Dilute the sample further
- Raise the solution temperature to boiling only
Correct Answer: Add a carrier to facilitate precipitation by co-precipitation
Q26. For metal sulfide precipitations, why is control of sulfide ion concentration and pH important?
- Because sulfide ions change the color of the solution only
- To maintain appropriate sulfide availability and avoid metal hydroxide formation or loss of sulfide as H2S gas
- pH has no effect on sulfide precipitation
- To promote oxidation of sulfide to sulfate
Correct Answer: To maintain appropriate sulfide availability and avoid metal hydroxide formation or loss of sulfide as H2S gas
Q27. What is post-precipitation and how does it affect results?
- It is the conversion of precipitate to oxide during ignition; it increases accuracy
- Precipitation occurring after filtration causing loss of analyte from filtrate and underestimation
- Intentional seeding to improve crystal size
- Evaporation of solvent after weighing
Correct Answer: Precipitation occurring after filtration causing loss of analyte from filtrate and underestimation
Q28. Which procedure reduces peptization and improves filterability of precipitates?
- Rapid stirring immediately after precipitation
- Digestion (aging) of the precipitate to allow particle growth and consolidation
- Using very fine filter paper only
- Washing with hot, pure water
Correct Answer: Digestion (aging) of the precipitate to allow particle growth and consolidation
Q29. Why is gravimetric analysis often considered highly accurate?
- Because it relies on direct mass measurement of a pure compound with known composition
- Because it needs only visual comparison
- Because color changes are always unambiguous
- Because it uses complex instruments that never need calibration
Correct Answer: Because it relies on direct mass measurement of a pure compound with known composition
Q30. After obtaining the mass of an ignited precipitate, what is the next calculation step?
- Report the precipitate mass as the analyte mass without conversion
- Use stoichiometry or the gravimetric factor to convert precipitate mass into analyte mass
- Divide the mass by the number of washings
- Multiply the mass by the solubility product constant
Correct Answer: Use stoichiometry or the gravimetric factor to convert precipitate mass into analyte mass
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