Which of the following is the general formula for saturated hydrocarbons?
- A.CₙH₂ₙ₊₂
- B.CₙH₂ₙ₋₂
- C.CₙH₂ₙ₊₁
- D.CₙH₂ₙ₋₁
Correct answer
Explanation
The correct answer is A, CₙH₂ₙ₊₂. Saturated open-chain hydrocarbons are the alkanes, and every alkane fits the formula CₙH₂ₙ₊₂.
In a saturated hydrocarbon every carbon atom is joined to others only by single bonds, so each carbon holds the largest possible number of hydrogen atoms. Putting n = 1 gives methane (CH₄), n = 2 ethane (C₂H₆) and n = 3 propane (C₃H₈). Alkanes are also called paraffins because they react little. Unsaturated hydrocarbons have double or triple bonds and therefore fewer hydrogen atoms for the same number of carbons.
B is wrong: CₙH₂ₙ₋₂ is the formula of the alkynes, such as ethyne (C₂H₂), which have a triple bond.
C is wrong: CₙH₂ₙ₊₁ is not a stable molecule but an alkyl group, such as methyl (CH₃–), formed by removing one hydrogen from an alkane.
D is wrong: CₙH₂ₙ₋₁ is not the formula of any hydrocarbon family; it fits a radical such as vinyl, not a saturated compound.
Exam tip: alkanes CₙH₂ₙ₊₂ (single bonds), alkenes CₙH₂ₙ (one double bond), alkynes CₙH₂ₙ₋₂ (one triple bond).
Practice Questions
View allSyngamy results in formation of
- A.haploid zygote
- B.diploid zygote
- C.non-motile male gametes
- D.motile male gametes
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Syngamy is the fusion of a male and a female gamete, and since each gamete is haploid, their union gives a diploid zygote.
Read the full explanation →Human insulin molecule is composed of one α-chain having 21 amino acids and one β-chain having 30 amino acids. How many functional insulin genes occur in adult humans?
- A.One
- B.Two
- C.Three
- D.Four
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Humans have a single functional insulin gene, INS, on the short arm of chromosome 11, and both chains of insulin come from it.
Read the full explanation →Which one of the following is the value of one nanometer?
- A.10⁻⁷ cm
- B.10⁻⁶ cm
- C.10⁻⁴ cm
- D.10⁻³ cm
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One nanometre is 10⁻⁹ metre, and since one metre is 100 (10²) centimetres, it equals 10⁻⁷ centimetre. The prefix nano means one billionth.
Read the full explanation →Which one of the following is not an epidemic disease?
- A.Cholera
- B.Malaria
- C.Smallpox
- D.Elephantiasis
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Elephantiasis, or lymphatic filariasis, is an endemic disease: it stays steadily present in certain regions and develops slowly over years, rather than breaking out suddenly in a large number of people.
Read the full explanation →The unit of the force constant k of a spring is
- A.N-m
- B.N/m
- C.N-m²
- D.N/m²
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The spring constant is force divided by extension, so its SI unit is newton per metre. Hooke's law says that, within the elastic limit, the restoring force of a spring is proportional to how far it is stretched or compressed: F = −kx.
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