Try this first
Answer: C. A group of compounds each containing a characteristic functional group forms a homologous series, and successive members differ in molecular formula by a CH₂ unit (NCERT Class 11 Chemistry, Chapter 8, page 262).
A is wrong: A is wrong because compounds with the same molecular formula and different structures are isomers, not members of one homologous series (trap: confusing homologues with isomers).
B is wrong: B is wrong because members of one series share a characteristic functional group and differ in carbon number (NCERT Class 11 Chemistry, Chapter 8, page 262).
D is wrong: D is wrong because a homologous series is defined by a functional group, not by a ring; the series NCERT lists include alkanes, alkenes and amines (NCERT Class 11 Chemistry, Chapter 8, page 262).
Methanol, ethanol and propanol are three different liquids, yet a chemist treats them as one family and predicts the properties of the third from the first. What makes them one family, and how can you tell in a few seconds whether two formulas belong to the same one?
Start with the part that does the work. The functional group is an atom or a group of atoms joined to the carbon chain which is responsible for the characteristic chemical properties of the organic compounds; examples are the hydroxyl group (—OH), the aldehyde group (—CHO) and the carboxylic acid group (—COOH) (NCERT Class 11 Chemistry, Chapter 8, page 262).
A group or a series of organic compounds, each containing a characteristic functional group, forms a homologous series, and the members of the series are called homologues (NCERT Class 11 Chemistry, Chapter 8, page 262).
Two tests decide membership. First, the members can be represented by a general molecular formula. Second, successive members differ from each other in molecular formula by a —CH₂ unit (NCERT Class 11 Chemistry, Chapter 8, page 262). Both tests need the same functional group, so a pair that differs by CH₂ but carries different functional groups is not a homologous pair.
NCERT names a number of these series: alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids and amines (NCERT Class 11 Chemistry, Chapter 8, page 262). Compounds with carbon and hydrogen only are hydrocarbons; a hydrocarbon with only carbon-carbon single bonds is saturated, and the IUPAC name for the homologous series of such compounds is alkane (NCERT Class 11 Chemistry, Chapter 8, page 263).
Watch out: "differ by CH₂" is not enough on its own. Check the functional group first, then count the CH₂ steps.
Given
The alkanoic acid series starts with HCOOH. CH₃COOH is the next member in the series.
Required
The molecular formula of the third member, and a check that it still belongs to the same series.
Concept
Successive members of a homologous series differ by a —CH₂ unit and keep the same functional group (NCERT Class 11 Chemistry, Chapter 8, page 262). The carboxylic acid group (—COOH) is the group that marks this series (NCERT Class 11 Chemistry, Chapter 8, page 262).
Formula
Next member = previous member + CH₂.
Substitution
HCOOH is CH₂O₂. CH₃COOH is CH₂O₂ + CH₂ = C₂H₄O₂. The third member is C₂H₄O₂ + CH₂.
Calculation
C₂H₄O₂ + CH₂ = C₃H₆O₂, which is CH₃CH₂COOH.
Final answer
The third member is C₃H₆O₂ (CH₃CH₂COOH). It keeps the —COOH group, so it is in the same series.
Common trap
Adding CH₃ instead of CH₂ gives C₃H₇O₂, which is not a formula for this series. Another slip is to add CH₂ but replace —COOH with another group, which leaves the series.
Similar NEET-style question
The alkanol series starts with CH₃OH. Write the molecular formula of its third member and say whether CH₃CHO belongs to the same series. (Answer: CH₃OH is CH₄O; CH₄O + 2 CH₂ = C₃H₈O, which is C₃H₇OH. CH₃CHO has —CHO, not —OH, so it is not in the series.)
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Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which description of a homologous series matches NCERT?
Answer: C. A group of compounds each containing a characteristic functional group forms a homologous series, and successive members differ in molecular formula by a CH₂ unit (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why A is wrong: A is wrong because compounds with the same molecular formula and different structures are isomers, not members of one homologous series (trap: confusing homologues with isomers).
Why B is wrong: B is wrong because members of one series share a characteristic functional group and differ in carbon number (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why D is wrong: D is wrong because a homologous series is defined by a functional group, not by a ring; the series NCERT lists include alkanes, alkenes and amines (NCERT Class 11 Chemistry, Chapter 8, page 262).
By what unit do the molecular formulas of successive members of a homologous series differ?
Answer: D. NCERT states that successive members of a homologous series differ from each other in molecular formula by a —CH₂ unit (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why A is wrong: A is wrong because adding CH₃ changes the hydrogen count by three and is not the repeating step of a series (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why B is wrong: B is wrong because —OH is a functional group, which stays the same along the series (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why C is wrong: C is wrong because CH₂CH₂ is two CH₂ units; successive members differ by one (trap: counting two steps as one).
What is the IUPAC name of the homologous series of hydrocarbons that contain only carbon-carbon single bonds?
Answer: C. A hydrocarbon is saturated if it contains only carbon-carbon single bonds, and the IUPAC name for the homologous series of such compounds is alkane (NCERT Class 11 Chemistry, Chapter 8, page 263).
Why A is wrong: A is wrong because alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond (NCERT Class 11 Chemistry, Chapter 8, page 263).
Why B is wrong: B is wrong because alkynes are unsaturated hydrocarbons with at least one carbon-carbon triple bond (NCERT Class 11 Chemistry, Chapter 8, page 263).
Why D is wrong: D is wrong because a haloalkane carries a halogen functional group and so is not a hydrocarbon (trap: treating any alkane derivative as an alkane).
Which pair belongs to the same homologous series?
Answer: B. CH₃OH and C₂H₅OH both carry the hydroxyl group (—OH) and differ by one CH₂ unit, so they are successive members of the alkanol series (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why A is wrong: A is wrong because CH₃OH has the —OH group and CH₃CHO has the —CHO group; different functional groups mean different series (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why C is wrong: C is wrong because C₂H₅OH has —OH and CH₃COOH has —COOH; the functional groups differ (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why D is wrong: D is wrong because the aldehyde group (—CHO) and the carboxylic acid group (—COOH) are different functional groups (trap: matching pairs by carbon count alone).
A compound has the aldehyde group (—CHO) as its functional group. Which homologous series from the NCERT list does it belong to?
Answer: D. NCERT lists the alkanals among the homologous series and gives the aldehyde group (—CHO) as a functional group example (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why A is wrong: A is wrong because alkanols are the series with the hydroxyl group (—OH) (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why B is wrong: B is wrong because alkanones are a different series from the alkanals in the NCERT list (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why C is wrong: C is wrong because alkanoic acids carry the carboxylic acid group (—COOH) (NCERT Class 11 Chemistry, Chapter 8, page 262).
Propan-1-ol (C₃H₇OH) and propanal (C₂H₅CHO) have the same number of carbon atoms. Why are they not members of the same homologous series?
Answer: A. Members of one series share a characteristic functional group; propan-1-ol has —OH and propanal has —CHO, so they belong to different series (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why B is wrong: B is wrong because the two compounds have the same number of carbon atoms, so they do not differ by a CH₂ unit in carbon count (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why C is wrong: C is wrong because each contains oxygen, so neither is a hydrocarbon (NCERT Class 11 Chemistry, Chapter 8, page 263).
Why D is wrong: D is wrong because the alkanals form a series that can be written with a general molecular formula (NCERT Class 11 Chemistry, Chapter 8, page 262).
The first member of the alkanol series is CH₃OH (molecular formula CH₄O). What is the molecular formula of the member two places after it?
Answer: B. Each step adds one CH₂ unit. Two steps add C₂H₄, so CH₄O becomes C₃H₈O (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why A is wrong: A is wrong because C₃H₆O adds only C₂H₂ to CH₄O, not two CH₂ units (trap: adding carbon without the matching hydrogens).
Why C is wrong: C is wrong because C₂H₈O adds only one carbon for two steps; two CH₂ units add two carbons (NCERT Class 11 Chemistry, Chapter 8, page 262).
Why D is wrong: D is wrong because C₃H₇O has an odd hydrogen count for this alcohol formula and does not equal CH₄O plus 2 CH₂ (trap: dropping a hydrogen from the formula).
In the alkane series, C₂H₆ is followed by C₃H₈. What is the molecular formula of the alkane four places after C₂H₆?
Answer: D. Four CH₂ units add C₄H₈ to C₂H₆, which gives C₆H₁₄ (NCERT Class 11 Chemistry, Chapter 8, page 262). The compound is an alkane because it has only carbon-carbon single bonds (NCERT Class 11 Chemistry, Chapter 8, page 263).
Why A is wrong: A is wrong because C₅H₁₂ is only three places after C₂H₆ (trap: miscounting the number of steps).
Why B is wrong: B is wrong because C₆H₁₂ has two hydrogens too few for the alkane series; it is the formula of an alkene or ring compound (NCERT Class 11 Chemistry, Chapter 8, page 263).
Why C is wrong: C is wrong because C₇H₁₆ is five places after C₂H₆ (trap: counting C₂H₆ itself as a step).
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The functional group is an atom or a group of atoms joined to the carbon chain which is responsible for the characteristic chemical properties of the organic compounds. The examples are hydroxyl group (—OH), aldehyde group (—CHO) and carboxylic acid group (— COOH) etc.
-- NCERT Class 11 Chemistry, Ch. 8, p. 262A group or a series of organic compounds each containing a characteristic functional group forms a homologous series and the members of the series are called homologues.
-- NCERT Class 11 Chemistry, Ch. 8, p. 262The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in molecular formula by a —CH2 unit.
-- NCERT Class 11 Chemistry, Ch. 8, p. 262There are a number of homologous series of organic compounds. Some of these are alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids, amines etc.
-- NCERT Class 11 Chemistry, Ch. 8, p. 262A hydrocarbon is termed saturated if it contains only carbon-carbon single bonds. The IUPAC name for a homologous series of such compounds is alkane.
-- NCERT Class 11 Chemistry, Ch. 8, p. 263No question in our NEET 2020–2025 set targets this topic directly.
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