Question 1

Which hydrocarbon shows the phenomenon of isomerism first appearing in its series?

Accepted Answer

Butane is the first alkane to exhibit structural isomerism, existing as n-butane and isobutane (2-methylpropane).

Question 2

What is the general molecular formula for alkanes?

Accepted Answer

Alkanes are saturated hydrocarbons with only single bonds, following the formula CₙH₂ₙ₊₂ for acyclic chains.

Question 3

Which of the following is an example of an alkene?

Accepted Answer

Ethene (C₂H₄) is an alkene containing one C=C double bond; alkenes follow the formula CₙH₂ₙ.

Question 4

How many sigma (σ) and pi (π) bonds are present in a molecule of ethyne (C₂H₂)?

Accepted Answer

Ethyne has a C≡C triple bond (1 σ and 2 π bonds between carbons) plus 2 C-H σ bonds, totaling 3 σ and 2 π bonds.

Question 5

What is the IUPAC name of the hydrocarbon with the structure CH₃-CH₂-CH₂-CH₃?

Accepted Answer

This four-carbon alkane with only single bonds is named butane according to IUPAC nomenclature.

Question 6

Which of the following is NOT an isomer of pentane (C₅H₁₂)?

Accepted Answer

2-methylpentane is C₆H₁₄ (hexane), not C₅H₁₂; the other three are all C₅H₁₂ isomers.

Question 7

Identify the general formula for alkenes.

Accepted Answer

Alkenes contain one C=C double bond and have the general formula CₙH₂ₙ.

Question 8

Which hydrocarbon shows the property of cis-trans isomerism?

Accepted Answer

2-butene (CH₃-CH=CH-CH₃) exhibits cis-trans isomerism due to restricted rotation around the C=C double bond.

Question 9

How many π (pi) bonds are present in 2-butyne (CH₃-C≡C-CH₃)?

Accepted Answer

2-butyne contains one triple bond (C≡C), which consists of 1 sigma and 2 pi bonds.

Question 10

What is the general molecular formula for alkanes?

Accepted Answer

Alkanes are saturated hydrocarbons with single C-C bonds, following the formula CₙH₂ₙ₊₂.

Question 11

Which hydrocarbon contains a triple bond between carbon atoms?

Accepted Answer

Ethyne (C₂H₂) is an alkyne with a C≡C triple bond, the simplest member of the alkyne series.

Question 12

How many sigma (σ) bonds are present in ethene (C₂H₄)?

Accepted Answer

Ethene has one C=C bond (1 sigma + 1 pi) and four C-H bonds, giving 5 sigma bonds total.

Question 13

What is the IUPAC name of the hydrocarbon CH₃-CH₂-CH₂-CH₃?

Accepted Answer

This straight-chain alkane has 4 carbon atoms, making it butane according to IUPAC nomenclature.

Question 14

Which of the following hydrocarbons exhibits geometric isomerism?

Accepted Answer

But-2-ene (CH₃CH=CHCH₃) has two different groups on each carbon of the double bond, allowing cis-trans isomerism; but-1-ene, butane, and but-1-yne do not.

Question 15

A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. Its molar mass is 56 g/mol. What is its molecular formula?

Accepted Answer

Empirical formula is CH₂ (mass ratio 12:2 = 85.7:14.3); empirical mass = 14; n = 56/14 = 4, so molecular formula is C₄H₈.

Question 16

When acetylene gas is passed through bromine water, the colour of bromine water fades and a white precipitate forms. What is the main organic product of this reaction?

Accepted Answer

Acetylene (HC≡CH) undergoes electrophilic addition with bromine to first form 1,2-dibromethene (CHBr=CHBr), which is the primary product under normal conditions.

AFCAT Hydrocarbons

Hydrocarbons — Practice Questions