State1 markBoth boards
Which is the general formula of the alkanes?
Show the mark scheme and the examiner note
Answer: CₙH₂ₙ₊₂
- Alkanes are saturated hydrocarbons
- Each carbon has four single bonds
- General formula CₙH₂ₙ₊₂
- A. CₙH₂ₙ₊₂ — Correct: alkanes are saturated, so each carbon carries the maximum hydrogen.
- B. CₙH₂ₙ — That is the alkenes, where the double bond costs two hydrogen atoms.
- C. CₙH₂ₙ₋₂ — That would be two hydrogens short, as in a compound with two degrees of unsaturation.
- D. CₙH₂ₙ₊₁OH — That is the alcohols, which have a hydroxyl group.
Examiner technique: Test a formula by its simplest member: methane is CH₄, and n = 1 gives CH₄ from CₙH₂ₙ₊₂.