Chemistry MCQs with answers Page - 136

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Q. Which reagent is commonly used to distinguish between aldehydes and ketones (Tollens' test)?

• (A) Tollen's reagent (ammoniacal Ag+)
• (B) 2,4-Dinitrophenylhydrazine (DNPH)
• (C) Bromine water
• (D) Fehling's solution

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Correct answer - Option (A)

Explanation:

Tollens' reagent (Ag(NH3)2+) oxidizes aldehydes to carboxylates and reduces Ag+ to metallic silver, producing a silver mirror — a positive test for aldehydes. DNPH reacts with both aldehydes and ketones to form hydrazones, so it doesn't distinguish them. Bromine water tests for unsaturation or phenols (and some active methylene compounds). Fehling's solution also distinguishes aldehydes (reducing) from many ketones, but Tollens' reagent is the classic answer here; Fehling's would also be acceptable in some contexts but not listed as the correct choice here.

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Q. Which of the following processes is endothermic?

• (A) Freezing of water
• (B) Condensation of steam
• (C) Sublimation of dry ice (CO2 solid → gas)
• (D) Formation of ionic lattice from gaseous ions

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Correct answer - Option (C)

Explanation:

Sublimation (solid to gas) requires input of energy — it's endothermic. Freezing and condensation release heat (exothermic). Formation of an ionic lattice from gaseous ions (lattice formation) releases energy (exothermic lattice energy). Hence sublimation is the correct endothermic process among the options.

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Q. What is the main type of intermolecular force responsible for the relatively high boiling point of water compared to H2S?

• (A) London dispersion forces
• (B) Dipole-dipole interactions
• (C) Hydrogen bonding
• (D) Ionic bonds

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Correct answer - Option (C)

Explanation:

Water exhibits strong hydrogen bonding (between H and highly electronegative O), which greatly raises its boiling point relative to H2S, which cannot hydrogen-bond effectively. London dispersion forces are present in all molecules but are not the dominant cause here. Dipole-dipole interactions are weaker than hydrogen bonds in this case. Ionic bonds are not relevant for neutral covalent molecules like water.

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Q. Which of the following correctly ranks species by increasing acidity in aqueous solution?

• (A) NH3 < H2O < HF
• (B) HF < H2O < NH3
• (C) H2O < HF < NH3
• (D) NH3 < HF < H2O

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Correct answer - Option (A)

Explanation:

In water, ammonia (NH3) is the weakest acid among these (pKa of NH4+ is about 9.25, so NH3 is very weak acid), water is intermediate (pKa ~15.7), and HF is stronger (pKa ~3.2) — so acidity increases NH3 < H2O < HF. The other options list incorrect orderings.

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Q. Which electrode reaction occurs at the cathode during the electrolysis of molten NaCl?

• (A) 2Cl- → Cl2 + 2e- (oxidation)
• (B) Na+ + e- → Na (reduction)
• (C) 2H2O + 2e- → H2 + 2OH-
• (D) Cl2 + 2e- → 2Cl- (reduction)

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Correct answer - Option (B)

Explanation:

In molten NaCl there is no water, only Na+ and Cl-. At the cathode cations are reduced: Na+ + e- → Na (metal). Option 1 is the oxidation of chloride that occurs at the anode, not the cathode. Option 3 describes reduction in aqueous electrolysis; not relevant for molten salt. Option 4 is reduction of chlorine gas, the reverse of the oxidation at the anode, not the cathodic process in this setup.

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Q. Which functional group is present in acetone (propanone)?

• (A) Aldehyde
• (B) Ketone
• (C) Alcohol
• (D) Carboxylic acid

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Correct answer - Option (B)

Explanation:

Acetone (CH3-CO-CH3) contains a carbonyl group bonded to two carbon atoms — this is a ketone. An aldehyde has a carbonyl bonded to at least one hydrogen. Alcohols have an -OH group. Carboxylic acids have -COOH. Therefore ketone is correct and the other options are incorrect functional groups for acetone.

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Q. Which law states that equal volumes of gases, at the same temperature and pressure, contain equal numbers of molecules?

• (A) Dalton's law
• (B) Avogadro's law
• (C) Boyle's law
• (D) Gay-Lussac's law

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Correct answer - Option (B)

Explanation:

Avogadro's law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules (or moles). Dalton's law concerns partial pressures. Boyle's law relates pressure and volume at constant temperature. Gay-Lussac's law relates pressure and temperature at constant volume (or in another form, volume and temperature at constant pressure depending on naming), but none of these describe equal numbers of molecules in equal volumes — that's Avogadro.

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Q. Which of the following is a characteristic of an ideal solution?

• (A) Volume changes on mixing
• (B) Heat is evolved or absorbed on mixing
• (C) Raoult's law is obeyed for all components
• (D) Components react chemically on mixing

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Correct answer - Option (C)

Explanation:

Ideal solutions obey Raoult's law across the composition range; there is no net enthalpy change on mixing (ΔHmix = 0) and no volume change on mixing (ΔVmix = 0). Therefore options 1 and 2 are characteristics of non-ideal solutions, and option 4 is false for ideal solutions because chemical reaction on mixing would make it non-ideal. Option 3 is the correct trait.

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Q. Which of the following species has the highest lattice energy?

• (A) NaCl
• (B) MgO
• (C) KBr
• (D) CsI

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Correct answer - Option (B)

Explanation:

Lattice energy increases with higher ionic charges and smaller ionic radii. MgO consists of Mg2+ and O2- (charges ±2), giving much larger lattice energy compared to salts of monovalent ions like NaCl, KBr, CsI. Therefore MgO has the highest lattice energy. KBr and CsI have larger ions and lower charges, making them much lower.

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Q. Which reagent is typically used to perform a nucleophilic substitution (SN2) reaction on a primary alkyl halide?

• (A) Tertiary amine (bulky base)
• (B) Sodium iodide in acetone
• (C) Peroxyacid
• (D) Chromic acid (H2CrO4)

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Correct answer - Option (B)

Explanation:

Sodium iodide in acetone (Finkelstein reaction) is a common method to substitute halides, and iodide is a good nucleophile for SN2 on primary alkyl halides. A bulky base favors E2 elimination (option1) especially on secondary/tertiary centers. Peroxyacid is used for epoxidation of alkenes (not SN2). Chromic acid is an oxidizing agent used to oxidize alcohols, not to perform SN2 substitutions.

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