Basics of Hydrocarbons – NDA Chemistry PYQs

NDA 2023-II

Q. 1.  Which one of the following is NOT an example of organic compound ?

(a) Ammonium cyanate

(b) Marsh gas

(c) Urea

(d) Cane sugar

Answer: (a) Ammonium cyanate

Explanation: Ammonium cyanate (NH₄OCN) is generally classified as an inorganic compound, though it is historically significant (Wöhler converted it to urea in 1828). Marsh gas (CH₄), urea (CO(NH₂)₂) and cane sugar (C₁₂H₂₂O₁₁) are organic compounds. Note: the classification of ammonium cyanate is sometimes debated. Concept Tested: Organic vs inorganic – ammonium cyanate is inorganic

NDA 2023-I

Q. 2.  Which of the hydrocarbons are arranged as per the increasing order of their boiling points ?

(a) Methane, Butane, Propane, Heptane

(b) Propane, Butane, Pentane, Octane

(c) Propane, Butane, Heptane, Methane

(d) Octane, Ethane, Methane, Propane

Answer: (b) Propane, Butane, Pentane, Octane

Explanation: Boiling point of alkanes increases with molecular mass. Propane (C₃, bp −42°C) < Butane (C₄, bp −1°C) < Pentane (C₅, bp 36°C) < Octane (C₈, bp 126°C). This is in increasing order of boiling points. Concept Tested: Boiling points of alkanes  – increase with chain length

NDA 2022-II

Q. 3.  Liquid vegetable oils are converted to solid margarine by the use of

(a) hydrogen gas

(b) chlorine gas

(c) carbon dioxide gas

(d) oxygen gas

Answer: (a) hydrogen gas

Explanation: Vegetable oils contain unsaturated fatty acids (C=C bonds). Hydrogenation with H₂ (using Ni catalyst) saturates these bonds, converting liquid oils to solid fats (margarine). Concept Tested: Hydrogenation  – converting liquid oils to solid margarine using H₂

Q. 4.  The number of structural isomers of pentane is

(a) 5

(b) 4

(c) 2

(d) 3

Answer: (d) 3

Explanation: Pentane (C₅H₁₂) has three structural isomers: n-pentane (straight chain), isopentane (2-methylbutane), and neopentane (2,2-dimethylpropane). Concept Tested: Structural isomers of pentane – 3 isomers

Q. 5.  C₄H₈ belongs to the homologous series of

(a) alkanes

(b) alkenes

(c) alkynes

(d) cycloalkanes

Answer: (b) alkenes

Explanation: C₄H₈ has the general formula CₙH₂ₙ, which is the formula for alkenes (olefins). Alkanes: CₙH₂ₙ₊₂. Alkynes: CₙH₂ₙ₋₂. C₄H₈ is butene (or cyclobutane, but (b) alkenes is the more straightforward answer for CₙH₂ₙ formula). Concept Tested: Molecular formula and homologous series – C₄H₈ is an alkene

NDA 2021-I

Q. 6.  Hydrogenation of vegetable oils using nickel catalyst is an example of

(a) Substitution reaction.

(b) Elimination reaction.

(c) Addition reaction.

(d) Free-radical polymerization.

Answer: (c) Addition reaction.

Explanation: Hydrogenation adds H₂ across C=C double bonds in unsaturated vegetable oils (using Ni catalyst), converting liquid unsaturated fats to solid saturated fats (margarine). This is an addition reaction – atoms are added across the double bond. Concept Tested: Hydrogenation of oils – addition reaction

NDA 2017-II

Q. 7.  The compound C₆H₁₂O₄ contains

(a) 22 atoms per mole

(b) twice the mass percent of H as compared to the mass percent of C

(c) six times the mass percent of C as compared to the mass percent of H

(d) thrice the mass percent of H as compared to the mass percent of O

Answer: (c) six times the mass percent of C as compared to the mass percent of H

Explanation: C₆H₁₂O₄: atoms per molecule = 6+12+4 = 22 – so (a) is about 22 atoms per molecule, and per mole would be 22 × Avogadro’s number. But let’s check %C/%H: %C = 72/148 × 100 = 48.6%. %H = 12/148 × 100 = 8.1%. Ratio %C/%H = 48.6/8.1 = 6. So (c) is correct. Concept Tested: Percentage composition and atom count – C₆H₁₂O₄

NDA 2016-I

Q. 8.  Which one of the following carbon compounds will not give a sooty flame ?

(a) Benzene

(b) Hexane

(c) Naphthalene

(d) Anthracene

Answer: (b) Hexane

Explanation: Sooty flames occur when aromatic hydrocarbons (high C:H ratio) burn with incomplete combustion. Hexane (C₆H₁₄) is a saturated aliphatic hydrocarbon with a lower C:H ratio and burns with a relatively clean, non-sooty flame. Benzene, naphthalene and anthracene are aromatic and produce sooty flames. Concept Tested: Sooty flames – aromatic hydrocarbons vs aliphatic

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