How many isomers can be formed from a compound with molecular formula C$_5$H$_{12}$ ?

Analyze the question: C₅H₁₂ is an alkane (CₙH₂ₙ₊₂ with n = 5). We need to count the different structural arrangements of the carbon atoms.

Concept: Isomers have the same molecular formula but different structural formulas. For alkanes, isomers arise from branching of the carbon chain.

Solution:

1. Straight chain: n-pentane CH₃–CH₂–CH₂–CH₂–CH₃

2. One methyl branch on a 4-carbon chain: 2-methylbutane (isopentane) CH₃–CH(CH₃)–CH₂–CH₃

3. Two methyl branches on a 3-carbon chain: 2,2-dimethylpropane (neopentane) CH₃–C(CH₃)₂–CH₃

Other arrangements, such as 3-methylbutane, are just 2-methylbutane numbered from the other end, so they are not new isomers.

So C₅H₁₂ has three structural isomers in total: n-pentane, 2-methylbutane, and 2,2-dimethylpropane.

Tip: Remember the pattern for alkane isomers: C₄H₁₀ has 2, C₅H₁₂ has 3, C₆H₁₄ has 5, C₇H₁₆ has 9. To find them, start with the straight chain, then shorten the main chain one carbon at a time and place the branches in every distinct position, avoiding duplicates.