Name the straight chain alkane having chemical formula C 9 H 20 , which is used as a research chemical and in making biodegradable detergents.

Correct — A. A straight-chain alkane obeys CnH2n+2.
Put n = 9: hydrogens = 2(9) + 2 = 20, so the formula is C9H20.
That is nonane — option (a).

The name is built from the numeral: non- for nine, plus the -ane ending that marks a saturated hydrocarbon.

Test the rest against the same rule. Nonadecane is n = 19, dodecane n = 12, icosane n = 20 — every one a longer chain than the formula allows.

Why the others are wrong

• (b)
  Nonadecane — Nonadecane is the nineteen-carbon alkane, C19H40. The shared syllable non- is the trap, since nonadeca- reads as nine-and-ten, that is nineteen.
• (c)
  Dodecane — Dodecane is the twelve-carbon alkane, C12H26, from do- for two plus deca- for ten. Twelve carbons, not nine.
• (d)
  Icosane — Icosane is the twenty-carbon alkane, C20H42, and is a waxy solid at room temperature rather than a liquid.

Alkanes are saturated hydrocarbons — single bonds only — and their molecular formula follows one rule, CnH2n+2.

Because the rule is fixed, name and formula are interchangeable. Read an IUPAC name as numeral plus ending: meth- 1, eth- 2, prop- 3, but- 4, then pent- 5, hex- 6, hept- 7, oct- 8, non- 9, dec- 10.

Beyond ten the prefixes combine — undec- 11, dodec- 12, and icos- 20. Straight chain means the carbons run in an unbranched line, which is what the n- or normal prefix marks.

Key facts

• Alkanes follow the general formula CnH2n+2.
• Nonane is C9H20 and dodecane is C12H26.
• Nonadecane has nineteen carbons and icosane has twenty.
• The prefix non- stands for nine and the ending -ane marks a saturated hydrocarbon.

Common traps

• Reading nonadecane as the nine-carbon alkane because it opens with non-.
• Counting hydrogens instead of carbons and picking a twenty-carbon chain.

No directly related past PYQ was found.