Questions & explanations
1. A colorless solution gives a white precipitate when treated with a few drops of ammonium hydroxide. The precipitate dissolves upon addition of excess ammonium hydroxide. Which of the following cations is most likely present?
- Al³⁺
- Zn²⁺
- Cu²⁺
- Fe³⁺
Answer: Zn²⁺
Zinc ions (Zn²⁺) form a white gelatinous precipitate of Zn(OH)₂ with dilute NH₄OH. This precipitate dissolves in excess NH₄OH due to the formation of the soluble complex [Zn(NH₃)₄]²⁺. Al³⁺ gives a white Al(OH)₃ precipitate that is insoluble in excess NH₄OH. Cu²⁺ gives a pale blue Cu(OH)₂ precipitate that dissolves in excess NH₄OH to form a deep blue complex, but the initial precipitate is not white. Fe³⁺ gives a reddish brown Fe(OH)₃ precipitate that is insoluble in excess NH₄OH.
2. Which of the following anions does NOT produce a gas when treated with dilute hydrochloric acid?
- Carbonate (CO₃²⁻)
- Sulphide (S²⁻)
- Nitrite (NO₂⁻)
- Sulphate (SO₄²⁻)
Answer: Sulphate (SO₄²⁻)
Dilute HCl reacts with carbonate to give CO₂, with sulphide to give H₂S, and with nitrite to give NO₂ (brown fumes). Sulphate does not evolve any gas with dilute HCl; Sulphate does not evolve any gas even with concentrated H₂SO₄; it is unreactive with both dilute and concentrated acids and is confirmed instead by precipitation reactions (e.g., white BaSO₄ precipitate with BaCl₂, insoluble in dilute HCl).. Hence sulphate is the correct choice.
3. A colourless salt gives a brown precipitate (Millon's base) on warming with Nessler's reagent. The cation present in the salt is:
- Na⁺
- K⁺
- NH₄⁺
- Mg²⁺
Answer: NH₄⁺
Nessler's reagent (an alkaline solution of K₂[HgI₄]) is the standard test for the ammonium ion. With NH₄⁺ it gives a brown precipitate (iodide of Millon's base), so among the cations listed only NH₄⁺ responds to Nessler's reagent. (Na⁺, K⁺ and Mg²⁺ give no reaction.) Note that the characteristic Nessler precipitate is brown, not white; the colour stated in the option stem does not change the fact that NH₄⁺ is the only correct choice.
4. Which of the following is the fundamental principle employed in the qualitative analysis of acid radicals (anions)?
- Anions are identified by the colour of their salts in solid state.
- Anions are classified and identified based on their reactions with dilute and concentrated acids to produce characteristic gases or precipitates.
- Anions are separated by precipitation with group reagents in the same order as cations.
- Anions are identified by flame test emission spectra.
Answer: Anions are classified and identified based on their reactions with dilute and concentrated acids to produce characteristic gases or precipitates.
In qualitative analysis of anions, the primary principle involves reacting the salt with dilute HCl (or dilute H₂SO₄) to test for gases like CO₂, H₂S, SO₂, NO₂, etc. For anions that do not react with dilute acids, concentrated H₂SO₄ is used to evolve specific gases. This classification and subsequent confirmatory tests rely on characteristic chemical reactions, not on physical colour or flame emission.
5. The preparation of ammonia (Haber's process) and ethanol (hydration of ethene) conceptually share which type of reaction?
- Substitution
- Addition
- Elimination
- Dehydration
Answer: Addition
Both processes involve addition of smaller molecules to form a larger product. In Haber process, nitrogen and hydrogen combine by addition to form ammonia. In hydration of ethene, water adds across the carbon-carbon double bond to yield ethanol. Substitution involves replacement of atoms, elimination removes atoms, and dehydration specifically removes water; none describe both reactions.
6. In the qualitative analysis of a mixture containing both nitrate and nitrite ions, which reagent is commonly added to remove the interference of nitrite before performing the brown ring test for nitrate?
- Dilute HCl
- Urea
- Barium chloride solution
- Ammonium hydroxide
Answer: Urea
Nitrite ions interfere with the brown ring test for nitrate because they also give a brown colour with FeSO₄ and conc. H₂SO₄. To destroy nitrite, urea is added along with dilute H₂SO₄; nitrite is decomposed to N₂ gas. Dilute HCl does not remove nitrite, BaCl₂ precipitates sulphate but not nitrite, and NH₄OH does not destroy nitrite. Hence urea is the correct choice.
7. On adding dilute hydrochloric acid to a solid salt, a colourless gas with a pungent odour is evolved. The gas turns acidified potassium dichromate solution green. Which anion is likely present?
- Carbonate (CO₃²⁻)
- Sulphide (S²⁻)
- Nitrite (NO₂⁻)
- Sulphite (SO₃²⁻)
Answer: Sulphite (SO₃²⁻)
Sulphite reacts with dilute HCl to release SO₂ gas, which is colourless, pungent, and reduces acidified K₂Cr₂O₇ (orange) to Cr³⁺ (green). Carbonate gives CO₂ which turns lime water milky but does not affect K₂Cr₂O₇. Sulphide gives H₂S (rotten egg odour) that turns lead acetate black. Nitrite gives brown fumes of NO₂. Thus sulphite is correct.
8. Which of the following tests is specifically used for the confirmatory identification of phosphate ion (PO₄³⁻)?
- Brown ring test
- Silver nitrate test
- Ammonium molybdate test
- Barium chloride test
Answer: Ammonium molybdate test
The ammonium molybdate test, performed by adding (NH₄)₂MoO₄ and conc. HNO₃, yields a canary‑yellow precipitate of ammonium phosphomolybdate, confirming phosphate. Brown ring test is for nitrate, silver nitrate test detects halides, and barium chloride test is for sulphate (white BaSO₄). Hence ammonium molybdate test is correct.
9. Which of the following is commonly used as a self-indicator in redox titrations?
- Potassium permanganate
- Potassium dichromate
- Iodine
- Starch
Answer: Potassium permanganate
Potassium permanganate (KMnO4) acts as a self-indicator because its intense purple colour disappears when reduced to colourless Mn2+ in acidic medium, allowing the end point to be detected without adding another indicator. Potassium dichromate is not self-indicating; an external indicator (e.g., diphenylamine) is needed.
10. In the systematic qualitative analysis of cations, the separation of cations into different groups is primarily based on:
- The difference in their atomic radii
- The difference in the solubility products of their salts with specific group reagents
- The difference in their ionization energies
- The difference in their oxidation states
Answer: The difference in the solubility products of their salts with specific group reagents
Systematic qualitative analysis relies on the fact that different cations form salts of varying solubility with group reagents. By using a common ion effect and controlled conditions (e.g., pH), the solubility product principle allows selective precipitation of one group after another, ensuring complete separation.
11. Which of the following is NOT a primary standard substance for preparing a standard solution?
- Oxalic acid
- Sodium carbonate
- Potassium permanganate
- Potassium hydrogen phthalate
Answer: Potassium permanganate
Potassium permanganate is not a primary standard because it decomposes on exposure to light, often contains impurities like MnO2, and its solutions are unstable. Oxalic acid, anhydrous sodium carbonate, and potassium hydrogen phthalate are stable, pure, and non-hygroscopic, making them suitable primary standards.
12. Which indicator is most suitable for the titration of a weak acid (acetic acid) with a strong base (NaOH)?
- Methyl orange
- Phenolphthalein
- Methyl red
- Bromothymol blue
Answer: Phenolphthalein
The equivalence point of a weak acid–strong base titration lies above pH 7 (around pH 8–9). Phenolphthalein changes colour in the pH range 8.3–10.0, so it is suitable. Methyl orange and methyl red change colour in the acidic range, while bromothymol blue changes near neutral pH; none is ideal for this titration.