Hydrocarbons, naming, isomers

Which of the following are classified as organic molecules?

• All compounds are organic.

• Compound II only

• Compounds II and IV only

• Compounds II, IV and V only

Answer explanation

Organic molecules contain at least one carbon atom (and usually hydrogen).

Which of the following lists includes alkenes only?

• C₂H₂, C₂H₄, C₂H₆

• C₂H₆, C₅H₁₀, C₆H₁₂

• CH₄, C₃H₈, C₇H₁₆

• C₃H₆, C₅H₁₀, C₆H₁₂

Answer explanation

Alkenes have the general formula C_nH_2n.

Which compound is a saturated hydrocarbon?

• Ethene

• Ethane

• Chloroethane

• Chloroethene

Answer explanation

Ethane. Hydrocarbons contain only carbon and hydrogen. Saturated hydrocarbons have only single bonds. Ethene is unsaturated, and chloroethane and chloroethene are a haloalkane and haloalkene, respectively.

What are the products formed when ethane reacts in excess oxygen?

• CO and H₂

• CO₂ and H₂

• CO and H₂O

• CO₂ and H₂O

Answer explanation

When a hydrocarbon reacts (combusts) in excess oxygen, the products are CO₂ and H₂O.

Which of the following alkanes has the highest boiling point?

• Methane

• Ethane

• Propane

• Butane

Answer explanation

All four options are alkanes, which are non-polar molecules. The only intermolecular force between these types of molecules is dispersion forces, which increase with larger molecules due to an increase in the number of electrons. Butane is the largest molecule (with a chain of 4 carbons) and therefore will have the highest boiling point.

What is the systematic name of the molecule shown?

• Propane

• Pentane

• Propene

• Pentene

Answer explanation

It is an alkane with a 5-carbon backbone: hence, pentane.

What is the systematic name of the compound with the condensed formula CH₃CH(CH₃)CH₂CH₂CH₃?

• hexane

• methyl pentane

• 2-methyl pentane

• 4-methyl hexane

Answer explanation

On the 2nd carbon atom (2-), there is a one-carbon branch (methyl) on a five-carbon (pent) singly bonded chain (ane).

Isomers of organic compounds have what?

• The same number of carbon atoms

• The same structural formula

• The same semi-structural formula

• The same molecular formula

Which of the following is not an isomer of the others?

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How many structural isomers have the formula C₄H₁₀?

• 1

• 2

• 3

• 4

Answer explanation