Assay methods for inorganic compounds (*) MCQs With Answer

Assay Methods for Inorganic Compounds MCQs With Answer offer B. Pharm students a focused pathway to master pharmaceutical analysis. This topic covers core analytical chemistry techniques used in pharmacopoeial assays of inorganic salts: acid–base titration, redox titration (permanganate, dichromate, iodimetry/iodometry, cerimetry), precipitation titration (argentometric methods: Mohr, Volhard, Fajans), complexometric titration (EDTA), and gravimetric analysis. You’ll learn about primary standards, indicators (starch, diphenylamine sulfonate, EBT, murexide, fluorescein), buffers, masking agents, pH control, stoichiometry, and pharmacopeial assay conditions for compounds like sodium chloride, copper sulfate, magnesium sulfate, zinc oxide, hydrogen peroxide, and sulfates. These MCQs reinforce calculation logic, endpoint recognition, and interference control. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which titration principle is commonly used for the pharmacopoeial assay of sodium chloride?

• Argentometric Mohr titration with potassium chromate indicator
• Volhard back-titration with thiocyanate
• Complexometric titration with EDTA
• Strong acid–strong base titration with phenolphthalein

Correct Answer: Argentometric Mohr titration with potassium chromate indicator

Q2. The optimal pH range for a sharp endpoint in the Mohr method is:

• 1.0–2.0
• 3.0–4.0
• 6.5–10.0
• Above 11.0

Correct Answer: 6.5–10.0

Q3. In the Volhard method, nitric acid medium is used primarily to:

• Increase the solubility of silver chloride
• Prevent precipitation of silver hydroxide/carbonate and sharpen the Fe(III)-thiocyanate endpoint
• Oxidize thiocyanate to sulfate
• Stabilize adsorption indicators

Correct Answer: Prevent precipitation of silver hydroxide/carbonate and sharpen the Fe(III)-thiocyanate endpoint

Q4. Which statement correctly distinguishes iodimetry from iodometry?

• Iodimetry titrates iodine with thiosulfate; iodometry uses iodine as titrant
• Iodimetry uses iodine as titrant; iodometry titrates liberated iodine with thiosulfate
• Both use iodine as titrant
• Both titrate iodine with permanganate

Correct Answer: Iodimetry uses iodine as titrant; iodometry titrates liberated iodine with thiosulfate

Q5. Regarding starch indicator in iodine titrations, the best practice is to:

• Add starch at the beginning and boil vigorously
• Add starch near the endpoint at temperature below 40 °C
• Avoid starch altogether
• Add starch only in alkaline medium

Correct Answer: Add starch near the endpoint at temperature below 40 °C

Q6. The preferred primary standard for standardizing sodium thiosulfate solution is:

• Potassium dichromate
• Potassium iodate (in presence of excess iodide and acid)
• Ferrous ammonium sulfate
• Sodium oxalate

Correct Answer: Potassium iodate (in presence of excess iodide and acid)

Q7. During standardization of KMnO4 with oxalic acid, the solution is warmed to about 60–70 °C to:

• Decrease the reaction rate
• Prevent precipitation of manganese dioxide
• Accelerate the slow redox reaction to obtain a sharp endpoint
• Stabilize oxalic acid against oxidation

Correct Answer: Accelerate the slow redox reaction to obtain a sharp endpoint

Q8. The n-factor of KMnO4 in acidic medium (Mn(VII) to Mn(II)) is:

• 1
• 2
• 3
• 5

Correct Answer: 5

Q9. A suitable internal indicator for dichromate titration of Fe(II) is:

• Starch
• N-phenylanthranilic acid
• Ferroin
• Eriochrome Black T

Correct Answer: N-phenylanthranilic acid

Q10. Why does potassium permanganate act as a self-indicator in acidic redox titrations?

• It forms a blue complex at endpoint
• Excess permanganate imparts a persistent faint pink color at endpoint
• It decolorizes at endpoint due to MnO2 formation
• It fluoresces under UV light

Correct Answer: Excess permanganate imparts a persistent faint pink color at endpoint

Q11. In EDTA titration for total hardness (Ca2+ + Mg2+), the typical buffer and indicator are:

• Acetate buffer pH 4 with methyl orange
• Ammonia–ammonium chloride buffer pH 10 with Eriochrome Black T
• Phosphate buffer pH 7 with phenolphthalein
• Borate buffer pH 9 with starch

Correct Answer: Ammonia–ammonium chloride buffer pH 10 with Eriochrome Black T

Q12. A common masking agent for Al(III) in complexometric titration is:

• Potassium cyanide
• Triethanolamine
• Thiourea
• Ascorbic acid

Correct Answer: Triethanolamine

Q13. Precipitation titration using adsorption indicators such as fluorescein is known as:

• Mohr method
• Volhard method
• Fajans method
• Karl Fischer method

Correct Answer: Fajans method

Q14. In the gravimetric assay of sulfate, the analyte is precipitated as:

• Calcium sulfate
• Barium sulfate
• Lead sulfate
• Strontium sulfate

Correct Answer: Barium sulfate

Q15. To avoid peptization of BaSO4 during washing in gravimetry, you should:

• Wash with copious cold distilled water only
• Wash with hot water containing a small amount of dilute hydrochloric acid
• Wash with ethanol
• Add concentrated ammonia to the wash

Correct Answer: Wash with hot water containing a small amount of dilute hydrochloric acid

Q16. In iodometry, which species is directly titrated with standard sodium thiosulfate?

• Iodide ion
• Iodate ion
• Iodine liberated from the reaction
• Triiodide indicator

Correct Answer: Iodine liberated from the reaction

Q17. Sodium thiosulfate solutions are best preserved by:

• Acidifying with hydrochloric acid
• Adding sodium carbonate and storing in amber bottles
• Adding potassium permanganate
• Refrigeration at 4 °C

Correct Answer: Adding sodium carbonate and storing in amber bottles

Q18. The pharmacopoeial assay of hydrogen peroxide often employs:

• Neutralization with NaOH using phenolphthalein
• Acidic KMnO4 titration
• Argentometric titration
• Gravimetric analysis

Correct Answer: Acidic KMnO4 titration

Q19. For assay of sodium bicarbonate to total neutralization, the preferred indicator is:

• Phenolphthalein
• Methyl orange
• Starch
• Ferroin

Correct Answer: Methyl orange

Q20. Volhard method is commonly applied as a back-titration in halide assays because:

• Silver thiocyanate is more soluble than silver halides
• Silver halides dissolve completely in nitric acid
• Insoluble AgCl forms first, then excess Ag+ is titrated indirectly with thiocyanate
• Thiocyante forms a colored precipitate with halides

Correct Answer: Insoluble AgCl forms first, then excess Ag+ is titrated indirectly with thiocyanate

Q21. Ferroin is best described as:

• An acid–base indicator for strong acid titrations
• A redox indicator used in cerimetry
• A precipitation indicator for Fajans method
• A metallochromic indicator for EDTA titrations

Correct Answer: A redox indicator used in cerimetry

Q22. Cerimetric titrations use which titrant and medium combination?

• Ce(IV) in sulfuric acid medium
• Ce(III) in acetic acid medium
• Ce(IV) in sodium hydroxide medium
• Ce(III) in hydrochloric acid medium

Correct Answer: Ce(IV) in sulfuric acid medium

Q23. A reliable primary standard for acidimetry among the following is:

• Sodium carbonate (dried)
• Hydrochloric acid
• Sodium hydroxide
• Ammonia solution

Correct Answer: Sodium carbonate (dried)

Q24. The stoichiometry for standardizing HCl with Na2CO3 is:

• Na2CO3 + HCl → NaHCO3 + NaCl
• Na2CO3 + 2 HCl → 2 NaCl + H2O + CO2
• 2 Na2CO3 + HCl → NaHCO3 + NaCl
• Na2CO3 + HCl → Na2CO3·HCl

Correct Answer: Na2CO3 + 2 HCl → 2 NaCl + H2O + CO2

Q25. CO2-free water is recommended in standardizing alkali solutions because:

• CO2 accelerates titration
• CO2 forms carbonate/bicarbonate that affects the endpoint and concentration
• CO2 improves indicator color change
• CO2 prevents adsorption of ions

Correct Answer: CO2 forms carbonate/bicarbonate that affects the endpoint and concentration

Q26. In the iodometric assay of copper sulfate, which solid phase forms on addition of iodide?

• Cu
• CuI
• Cu2O
• Cu(OH)2

Correct Answer: CuI

Q27. The endpoint in a starch–iodine titration is recognized by:

• Appearance of red-brown color
• Disappearance of blue color of the starch–iodine complex
• Formation of white precipitate
• Emergence of green fluorescence

Correct Answer: Disappearance of blue color of the starch–iodine complex

Q28. A commonly used adsorption indicator in Fajans titration of chloride is:

• Diphenylamine sulfonate
• Fluorescein or dichlorofluorescein
• Starch
• Methyl orange

Correct Answer: Fluorescein or dichlorofluorescein

Q29. For complexometric assay of magnesium sulfate, the typical conditions are:

• pH 10 with Eriochrome Black T
• pH 4 with methyl orange
• pH 12 with murexide
• pH 7 with phenolphthalein

Correct Answer: pH 10 with Eriochrome Black T

Q30. Zinc oxide is often assayed complexometrically using:

• EBT at pH 10
• Xylenol orange at pH 5–6
• Murexide at pH 12
• Starch at pH 3

Correct Answer: Xylenol orange at pH 5–6

Q31. The primary role of a buffer in EDTA titration is to:

• Increase ionic strength only
• Maintain constant pH to stabilize metal–EDTA complex and indicator behavior
• Prevent precipitation by complexing EDTA
• Provide a redox couple

Correct Answer: Maintain constant pH to stabilize metal–EDTA complex and indicator behavior

Q32. A suitable primary standard for standardizing silver nitrate is:

• Sodium chloride (dried at 140 °C)
• Potassium permanganate
• Sodium thiosulfate
• Hydrochloric acid

Correct Answer: Sodium chloride (dried at 140 °C)

Q33. The endpoint in the Mohr method is detected by formation of:

• Blue starch–iodine complex
• Red-brown silver chromate after all chloride is precipitated
• Blood-red ferric thiocyanate complex
• Pink color due to excess permanganate

Correct Answer: Red-brown silver chromate after all chloride is precipitated

Q34. Among common oxidants in acidic medium, the strongest is generally:

• Potassium dichromate
• Potassium permanganate
• Ceric sulfate
• Hydrogen peroxide

Correct Answer: Potassium permanganate

Q35. The stoichiometric equation generating iodine from iodate and iodide in acid is:

• IO3− + I− → I2
• IO3− + 5 I− + 6 H+ → 3 I2 + 3 H2O
• IO3− + 3 I− + 6 H+ → 2 I2 + 3 H2O
• 2 IO3− + 2 I− + 6 H+ → 2 I2 + 3 H2O

Correct Answer: IO3− + 5 I− + 6 H+ → 3 I2 + 3 H2O

Q36. A classic primary standard for standard iodine solution in iodimetry is:

• Arsenic trioxide
• Sodium chloride
• Potassium permanganate
• Oxalic acid

Correct Answer: Arsenic trioxide

Q37. The endpoint in Volhard titration is detected by appearance of:

• Blue starch–iodine color
• Red-brown Ag2CrO4 precipitate
• Blood-red Fe(III)–thiocyanate complex
• Green ferroin complex

Correct Answer: Blood-red Fe(III)–thiocyanate complex

Q38. Nitric acid is preferred over hydrochloric acid in Volhard titrations because:

• Chloride would interfere by forming additional AgCl
• Nitric acid is weaker
• Nitric acid forms AgNO3 precipitate
• Thiocyante decomposes in HCl

Correct Answer: Chloride would interfere by forming additional AgCl

Q39. The assay of sulfate in sodium sulfate is typically performed by:

• EDTA titration at pH 10
• Gravimetric precipitation as BaSO4
• Volhard titration
• Iodometric titration

Correct Answer: Gravimetric precipitation as BaSO4

Q40. In gravimetric analysis of barium sulfate, ignition to constant weight is usually done at about:

• 100–150 °C
• 250–300 °C
• 500–600 °C
• 800–900 °C

Correct Answer: 800–900 °C

Q41. Nitrobenzene is used in the Volhard method to:

• Oxidize thiocyanate
• Coat AgCl to prevent dissolution and adsorption errors during back-titration
• Complex Fe(III)
• Buffer the solution to pH 10

Correct Answer: Coat AgCl to prevent dissolution and adsorption errors during back-titration

Q42. For mixtures of halides where direct endpoints are indistinct, the most suitable argentometric approach is:

• Mohr direct titration
• Volhard back-titration
• Fajans direct titration
• Potentiometric titration with glass electrode

Correct Answer: Volhard back-titration

Q43. A key property of EDTA complexes with metal ions is that they are typically:

• 2:1 metal:ligand complexes
• 1:2 metal:ligand complexes
• 1:1 metal:ligand complexes regardless of metal charge
• Variable stoichiometry depending on pH

Correct Answer: 1:1 metal:ligand complexes regardless of metal charge

Q44. For determining calcium alone in presence of magnesium by EDTA titration, typical conditions are:

• pH 7 with phenolphthalein
• pH 10 with EBT
• pH ~12 with murexide indicator (Mg(OH)2 precipitated)
• pH 4 with methyl orange

Correct Answer: pH ~12 with murexide indicator (Mg(OH)2 precipitated)

Q45. In acidic permanganate titrations, manganese is reduced from Mn(VII) to:

• Mn(IV)
• Mn(II)
• Mn(III)
• Mn(0)

Correct Answer: Mn(II)

Q46. An external indicator used for detecting the endpoint in Fe(II) titration with K2Cr2O7 is:

• Potassium ferricyanide on a spotting tile
• Starch solution
• Phenolphthalein
• Ferroin

Correct Answer: Potassium ferricyanide on a spotting tile

Q47. To minimize blank error due to dissolved oxygen in iodometric titrations, use:

• Freshly boiled and cooled water
• Oxygen-saturated water
• Strongly alkaline medium
• Excess nitric acid

Correct Answer: Freshly boiled and cooled water

Q48. In acidic medium, the stoichiometric relation between KMnO4 and Fe(II) is:

• 1 mol KMnO4 oxidizes 2 mol Fe(II)
• 1 mol KMnO4 oxidizes 3 mol Fe(II)
• 1 mol KMnO4 oxidizes 5 mol Fe(II)
• 1 mol KMnO4 oxidizes 10 mol Fe(II)

Correct Answer: 1 mol KMnO4 oxidizes 5 mol Fe(II)

Q49. A practical primary standard for standardizing EDTA is:

• High-purity calcium carbonate (dried), dissolved in acid and back-neutralized
• Sodium hydroxide pellets
• Hydrochloric acid
• Ammonia solution

Correct Answer: High-purity calcium carbonate (dried), dissolved in acid and back-neutralized

Q50. The pharmacopoeial assay of ammonium chloride typically involves:

• Direct titration with silver nitrate using Mohr method
• Iodometric titration with sodium thiosulfate
• Distillation of liberated ammonia into standard acid followed by back-titration
• Gravimetric precipitation as ammonium oxalate

Correct Answer: Distillation of liberated ammonia into standard acid followed by back-titration