Which of the following is a saturated hydrocarbon?

Correct — B. A saturated hydrocarbon has only single bonds. An open-chain one, an alkane, follows CnH2n+2.

For C₃H₈:
n = 3, so 2n + 2 = 8
The hydrogens match — this is propane, option (b).

Every carbon valency in propane is filled by a single bond, so it can take up no more hydrogen. Each of the other three formulas falls short of 2n + 2.

Why the others are wrong

• (a)
  C₂H₄ — C₂H₄ is ethene. Two carbons would need six hydrogens to be saturated; with four, the carbons share a double bond.
• (c)
  C₆H₆ — C₆H₆ is benzene. As an alkane six carbons would carry fourteen hydrogens; benzene has six, and its ring is unsaturated.
• (d)
  C₄H₈ — C₄H₈ fits CnH2n, the alkene formula, and NCERT lists it in the alkene series as butene. It is two hydrogens short of butane, C₄H₁₀.

Carbon compounds linked only by single bonds are saturated. Those with a double or triple bond between carbons are unsaturated, and more reactive.

Saturated hydrocarbons are alkanes, CnH2n+2. Alkenes, with one double bond, are CnH2n. Alkynes, with one triple bond, are CnH2n−2.

The quick test is to compare the hydrogen count with 2n + 2. Equal means saturated. Fewer means a double bond, a triple bond or a ring.

Key facts

• Alkanes are saturated hydrocarbons with the general formula CnH2n+2.
• Propane, C₃H₈, is the three-carbon alkane.
• Alkenes follow CnH2n: ethene is C₂H₄ and butene is C₄H₈.
• Benzene, C₆H₆, is an unsaturated cyclic hydrocarbon.

Common traps

• Assuming every ring compound is saturated, when benzene is a ring and unsaturated.
• Reading CnH2n as saturated, when it is the alkene formula unless the compound is a ring.

No directly related past PYQ was found.