What is Homologous series?

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Q. What is Homologous series?

Tamil Nadu DgeTamil Nadu HSC First Year (DGE) Board 2024Subjective· 2mImportance★★★★★

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A homologous series is a family of compounds with the same general formula and similar chemistry, where consecutive members differ by one -CH2- unit.

A homologous series is a series of organic compounds that share the following characteristics:

Same general formula: e.g. alkanes follow CnH2n+2, alkenes follow CnH2n, primary alcohols follow CnH2n+1OH.
Each member differs from the next by a common structural unit, -CH2- (a difference of 14 mass units, i.e. 1 carbon + 2 hydrogens), e.g. methane (CH4), ethane (C2H6), propane (C3H8), and so on.
Similar chemical properties: since all members share the same functional group, they undergo broadly similar characteristic reactions.
Gradual, regular variation in physical properties: melting point, boiling point, density, etc. change smoothly and progressively as molecular mass increases along the series (e.g. boiling points of alkanes rise steadily from methane to higher homologues). …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set A1 markMCQ
$Q. The general formula of ether is$
(a) CnH2nO
(b) CnH2n+1O
(c) CnH2n+2O
(d) CnH2nO2
›Reveal solutionSolution

Ethers (R-O-R') are functional isomers of monohydric alcohols, so both share the general formula CnH2n+2O.

An ether has the structure R-O-R' with only single bonds and no rings, i.e. it is fully saturated. Check with the simplest: dimethyl ether is CH3-O-CH3 = C2H6O, which fits CnH2n+2O for n = 2 (2x2+2 = 6). This is the same …

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CBSE 2025Set D1 markMCQ
$Q. The general formula of an amine is$
(a) CnH2n+1N
(b) CnH2n+2N
(c) CnH2n+3N
(d) CnH2nN
›Reveal solutionSolution

A saturated amine has the general formula CnH2n+3N.

A saturated aliphatic hydrocarbon is CnH2n+2. An amine is formed by replacing one hydrogen with an -NH2 (or by attaching nitrogen bearing hydrogens). Adding nitrogen with its extra hydrogens raises the hydrogen count by one relative to the alkane: …

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CBSE 2025Set D1 markMCQ
$Q. The general formula of carbonyl compound is$
(a) CnH2nO
(b) CnH2n+2O
(c) CnH2n+1O
(d) none of these
›Reveal solutionSolution

Saturated aldehydes and ketones (carbonyl compounds) share the general formula CnH2nO.

Carbonyl compounds are aldehydes (R-CHO) and ketones (R-CO-R'). For saturated members, the general molecular formula is CnH2nO. For example: HCHO (n=1, CH2O), CH3CHO (n=2, C2H4O), CH3COCH3 (n=3, C3H6O) — each fits CnH2nO. …

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CBSE 2025Set sz1 markMCQ
$Q. Select the correct one: Which of the following represents the general formula of alkenes?$
(a) CnH2n
(b) CnH2n-2
(c) CnH2n+2
(d) CnH2n+1
›Reveal solutionSolution

The general formula of alkenes (one C=C double bond) is CnH2n.

Saturated open-chain hydrocarbons (alkanes) have the general formula CnH2n+2. Each degree of unsaturation (one double bond or one ring) removes 2 hydrogen atoms from this formula. Alkenes contain exactly one C=C double bond, so their general formula is CnH2n+2 - 2 = CnH2n. (CnH2n-2 is the formula for alkynes/diene …

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CBSE 2025Set ANNUAL1 markMCQ
$Q. The general formula of alkene is$
(a) CnH2n+1
(b) CnH2n+2
(c) CnH2n-1
(d) CnH2n
›Reveal solutionSolution
Alkenes (one C=C double bond, one degree of unsaturation) follow CnH2n.

The general formulas of the common acyclic hydrocarbon families are:

Alkanes (saturated, no double/triple bonds): $C_nH_{2n+2}$

Alkenes (one C=C double bond): $C_nH_{2n}$

Alkynes (one C≡C triple bond): $C_nH_{2n-2}$ …
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CBSE 2024Set D1 markMCQ
$Q. The general molecular formula of saturated monocarboxylic acids is$
(a) CnH2n+2O
(b) CnH2nO
(c) CnH2nO2
(d) CnH2n+1O2
›Reveal solutionSolution
Saturated monocarboxylic acids: CnH2nO2.

A saturated monocarboxylic acid has one -COOH group. Checking with examples:

Formic acid HCOOH → CH2O2 (n=1): C1H2O2 fits CnH2nO2.

Acetic acid CH3COOH → C2H4O2 (n=2): fits CnH2nO2.

Propionic acid C2H5COOH → C3H6O2 (n=3): fits. …
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CBSE 2022Set ANNUAL1 mark
$Q. The formula of Alkene is ................. (Fill in the blank)$
›Reveal solutionSolution

The general formula of an alkene is CnH2n.

Alkenes are unsaturated hydrocarbons containing one carbon-carbon double bond (C=C). Because of the double bond, an alkene has two fewer hydrogen atoms than the corresponding saturated alkane (CnH2n …

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CBSE 2022Set ANNUAL1 markMCQ
$Q. CnH2n-2 is general formula of :$
(a) Alkanes
(b) Arenes
(c) Alkenes
(d) Alkynes
›Reveal solutionSolution
Each degree of unsaturation removes 2 H atoms from the saturated C $_n$ H $_{2n+2}$ formula; a triple bond (2 degrees) gives C $_n$ H $_{2n-2}$ — the alkyne formula.

Saturated alkanes follow C $_n$ H $_{2n+2}$ (no unsaturation). Each ring or π-bond reduces the H count by 2:

Alkenes (one C=C double bond, one degree of unsaturation): C $_n$ H $_{2n}$ .

Alkynes (one C≡C triple bond, two degrees of unsaturation): C $_n$ H $_{2n-2}$ . …
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CBSE 2021Set A1 markMCQ
$Q. The general formula of monohydric alcohol is$
(a) CnH2n+1OH
(b) CnH2n+2OH
(c) CnH2nOH
(d) None of these
›Reveal solutionSolution

Monohydric alcohol = alkyl-OH = CnH2n+1OH.

A monohydric alcohol contains one hydroxyl (-OH) group attached to a saturated alkyl group. A saturated alkyl group has the formula CnH2n+1 (e.g. CH3-, C2H5-). Attaching -OH gives the general formula CnH2n+1OH, e …

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CBSE 2021Set ANNUAL1 markMCQ
$Q. CnH2n is general formula of:$
(a) alkene
(b) alkane
(c) alkyne
(d) arene
›Reveal solutionSolution
Alkenes are unsaturated hydrocarbons with one C=C double bond, giving the general formula $C_nH_{2n}$ — two hydrogens fewer per molecule than the corresponding alkane ( $C_nH_{2n+2}$ ).

Step 1 — Compare the hydrocarbon families:

Alkanes (saturated, all single bonds): $C_nH_{2n+2}$ — e.g. ethane $C_2H_6$ .

Alkenes (one C=C double bond): $C_nH_{2n}$ — e.g. ethene $C_2H_4$ .

Alkynes (one C≡C triple bond): $C_nH_{2n-2}$ — e.g. ethyne $C_2H_2$ . …
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Q.What is Homologous series?

Concept understanding — Homologous Series and General Formulas

Q.What is Homologous series?

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