Polythene is obtained by the combination of large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst. The large molecules thus obtained are called polymers. This reaction is known as polymerisation.
-- NCERT Class 11 Chemistry, Ch. 9, p. 313Alkene Polymerization
8 MCQs9-step worked example
Source: NCERT HydrocarbonsOfficial key: NTA-verifiedLast updated: 8 Oct 2026
Try this first
Polythene is obtained from ethene under which conditions?
- A.Low temperature and low pressure with no catalyst
- B.High temperature, high pressure and a catalyst
- C.Room temperature with sunlight only
- D.High temperature with excess water
Tap to see the answer
Answer: B. Polythene is obtained by the combination of a large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst (NCERT Class 11 Chemistry, Chapter 9, page 313).
A is wrong: A is wrong because NCERT names high temperature, high pressure and a catalyst, not mild conditions (NCERT Class 11 Chemistry, Chapter 9, page 313).
C is wrong: C is wrong because NCERT does not give sunlight as the condition; polymerisation of ethene needs high temperature, high pressure and a catalyst (NCERT Class 11 Chemistry, Chapter 9, page 313).
D is wrong: D is wrong because water is not a reagent in this reaction; the ethene molecules combine with one another (NCERT Class 11 Chemistry, Chapter 9, page 313).
Alkene Polymerization, explained for NEET
The question to answer first: the plastic bag in your hand is made of an alkene. Which one, and how do small molecules turn into a bag?
Polythene is obtained by the combination of a large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst. The large molecules thus obtained are called polymers, and the reaction is known as polymerisation (NCERT Class 11 Chemistry, Chapter 9, page 313).
The simple compounds from which polymers are made are called monomers. Ethene is the monomer of polythene. Other alkenes also undergo polymerisation, for example propene, which gives polypropene (NCERT Class 11 Chemistry, Chapter 9, page 314).
In the equation, n molecules of the monomer join to give a chain of n repeating units: n(CH₂=CH₂) gives –(CH₂–CH₂)ₙ– and n(CH₃–CH=CH₂) gives the polypropene chain (NCERT Class 11 Chemistry, Chapter 9, page 314). The double bond of each monomer is used up in joining the units, so the chain has no C=C left.
NCERT lists what the polymers are used for: plastic bags, squeeze bottles, refrigerator dishes, toys, pipes, radio and T.V. cabinets. Polypropene is used for milk crates, plastic buckets and other moulded articles (NCERT Class 11 Chemistry, Chapter 9, page 314).
NCERT closes with a warning: though these materials have now become common, excessive use of polythene and polypropylene is a matter of great concern for all of us (NCERT Class 11 Chemistry, Chapter 9, page 314).
Watch out: the monomer is the alkene with the double bond, not the polymer. A question that shows the repeating unit –(CH₂–CH₂)ₙ– is asking you to put the double bond back to find the monomer.
How do you solve a Alkene Polymerization question? A worked example
- 1
Given
2.8 × 10² g of ethene (C₂H₄, molar mass 28 g mol⁻¹) is polymerised completely to polythene.
- 2
Required
The number of moles of ethene used and the mass of polythene formed.
- 3
Concept
Polythene is obtained by the combination of a large number of ethene molecules, written n(CH₂=CH₂) giving –(CH₂–CH₂)ₙ–. All atoms of the monomers stay in the chain (NCERT Class 11 Chemistry, Chapter 9, page 313).
- 4
Formula
Moles = mass ÷ molar mass; mass of polymer = mass of monomer, because every monomer unit becomes one repeating unit.
- 5
Substitution
Moles of ethene = (2.8 × 10² g) ÷ (28 g mol⁻¹)
- 6
Calculation
(2.8 × 10²) ÷ 28 = 10 mol, which is 1.0 × 10¹ mol to two significant figures. The number of repeating units equals the moles of ethene used. The chain keeps all the carbon and hydrogen of the monomers, so the mass is unchanged. The count of units and the equality of the masses are exact relations and do not limit the significant figures.
- 7
Final answer
1.0 × 10¹ mol of ethene is used, and the polythene formed has mass 2.8 × 10² g.
- 8
Common trap
Writing 28 g because the mass of one ethene molecule is 28 g mol⁻¹. That is the mass of one mole of units, not of the whole sample.
- 9
Similar NEET-style question
How many grams of propene (42 g mol⁻¹) must be polymerised completely to obtain 8.4 × 10¹ g of polypropene? (Answer: 8.4 × 10¹ g, since the polymer holds all the atoms of the monomers; that is 2.0 mol of propene units.)
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Can you answer these Alkene Polymerization MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Polythene is obtained from ethene under which conditions?
Show answer and why every option is right or wrong
Answer: B. Polythene is obtained by the combination of a large number of ethene molecules at high temperature, high pressure and in the presence of a catalyst (NCERT Class 11 Chemistry, Chapter 9, page 313).
Why A is wrong: A is wrong because NCERT names high temperature, high pressure and a catalyst, not mild conditions (NCERT Class 11 Chemistry, Chapter 9, page 313).
Why C is wrong: C is wrong because NCERT does not give sunlight as the condition; polymerisation of ethene needs high temperature, high pressure and a catalyst (NCERT Class 11 Chemistry, Chapter 9, page 313).
Why D is wrong: D is wrong because water is not a reagent in this reaction; the ethene molecules combine with one another (NCERT Class 11 Chemistry, Chapter 9, page 313).
MCQ 2Easy RecallPractice
In polymerisation, the simple compounds from which a polymer is made are called
Show answer and why every option is right or wrong
Answer: C. The simple compounds from which polymers are made are called monomers (NCERT Class 11 Chemistry, Chapter 9, pages 313 and 314).
Why A is wrong: A is wrong because a catalyst is a separate requirement of the reaction; it is not the compound that is joined into the chain (NCERT Class 11 Chemistry, Chapter 9, page 313).
Why B is wrong: B is wrong because the large molecules obtained are the polymers; the simple starting compounds are the monomers (NCERT Class 11 Chemistry, Chapter 9, page 313).
Why D is wrong: D is wrong because isomers are compounds with the same formula but different structures, which is a different idea from the units of a polymer (NCERT Class 11 Chemistry, Chapter 9, page 314).
MCQ 3Easy RecallPractice
Which statement about the use of polymers is given in NCERT?
Show answer and why every option is right or wrong
Answer: A. Polypropene is used for the manufacture of milk crates, plastic buckets and other moulded articles (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why B is wrong: B is wrong because NCERT names squeeze bottles among the general uses of polymers and gives milk crates and buckets for polypropene; the word only is not supported (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why C is wrong: C is wrong because NCERT lists plastic bags, bottles, dishes, toys, pipes and cabinets as uses, not making metals lighter (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why D is wrong: D is wrong because NCERT states that the excessive use of polythene and polypropylene is a matter of great concern (NCERT Class 11 Chemistry, Chapter 9, page 314).
MCQ 4Direct ApplicationPractice
Which of the following can act as a monomer in the polymerisation reactions described for alkenes?
Show answer and why every option is right or wrong
Answer: D. Alkenes undergo polymerisation, and propene, CH₃–CH=CH₂, polymerises to polypropene (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why A is wrong: A is wrong because ethane is an alkane with no double bond; the polymerisation described by NCERT is that of alkenes (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why B is wrong: B is wrong because propane is saturated and has no C=C to open; the monomers named are alkenes (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why C is wrong: C is wrong because methane is saturated and has no double bond; only alkenes are said to polymerise in this way (NCERT Class 11 Chemistry, Chapter 9, page 314).
MCQ 5Direct ApplicationPractice
The repeating unit of a polymer is –(CH₂–CH₂)–. Which monomer was polymerised?
Show answer and why every option is right or wrong
Answer: C. n(CH₂=CH₂) gives –(CH₂–CH₂)ₙ–, which is polythene, so the monomer is ethene (NCERT Class 11 Chemistry, Chapter 9, pages 313 and 314).
Why A is wrong: A is wrong because ethane has no double bond and is not named as a monomer; polythene comes from ethene (NCERT Class 11 Chemistry, Chapter 9, page 313).
Why B is wrong: B is wrong because propene gives the polypropene chain, whose repeating unit carries a CH₃ group on every second carbon (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why D is wrong: D is wrong because ethyne is an alkyne; the polymerisation of ethene described here gives polythene (NCERT Class 11 Chemistry, Chapter 9, page 313).
MCQ 6Direct ApplicationPractice
A student states: "Polymerisation of an alkene is a reaction of the double bond, so the chain obtained still contains the C=C bonds of the monomers." What is the best response?
Show answer and why every option is right or wrong
Answer: D. The equation n(CH₂=CH₂) giving –(CH₂–CH₂)ₙ– shows that the double bond of each ethene unit is used in joining the units, so the polymer chain has no C=C (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why A is wrong: A is wrong because polythene, –(CH₂–CH₂)ₙ–, has only single bonds in the chain, so polymers of alkenes are not unsaturated in this way (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why B is wrong: B is wrong because the monomers are alkenes; it is the double bond that lets the molecules combine (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why C is wrong: C is wrong because the page describes a one-way combination of monomers into polymers and says nothing about ethene being released again (NCERT Class 11 Chemistry, Chapter 9, page 313).
MCQ 7CalculationPractice
An ideal polypropene chain is built from exactly 250 propene molecules (C₃H₆, molar mass 42 g mol⁻¹). Ignoring end groups, what is the molar mass of the chain?
Show answer and why every option is right or wrong
Answer: A. The chain contains 250 repeating units –(CH(CH₃)–CH₂)– and each unit has the formula and mass of one propene molecule (NCERT Class 11 Chemistry, Chapter 9, page 314). So the molar mass is 250 × 42 = 10 500 g mol⁻¹ = 1.05 × 10⁴ g mol⁻¹.
Why B is wrong: B is wrong because 42 g mol⁻¹ is the mass of one propene molecule, a single unit; the chain has 250 of them (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why C is wrong: C is wrong because 6.0 results from dividing 250 by 42 (about 5.95) instead of multiplying them (common slip: dividing instead of multiplying).
Why D is wrong: D is wrong because 250 is the number of units, not a mass; it must be multiplied by the unit mass (common slip: reporting the count).
MCQ 8CalculationPractice
Two alkenes P and Q polymerise to give the chains –(CH₂–CH₂)ₙ– and –(CH(CH₃)–CH₂)ₙ– respectively. Which pair correctly names P and Q and their polymers?
Show answer and why every option is right or wrong
Answer: B. n(CH₂=CH₂) gives polythene and n(CH₃–CH=CH₂) gives polypropene, so P is ethene giving polythene and Q is propene giving polypropene (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why A is wrong: A is wrong because it swaps the two: the chain with the CH₃ branch comes from propene and the unbranched chain from ethene (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why C is wrong: C is wrong because ethane and propane are alkanes with no double bond; the monomers are the alkenes ethene and propene (NCERT Class 11 Chemistry, Chapter 9, page 314).
Why D is wrong: D is wrong because it names the right monomers but the wrong polymers: ethene gives polythene and propene gives polypropene (NCERT Class 11 Chemistry, Chapter 9, page 314).
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What to remember before solving Alkene Polymerization questions
4 NCERT lines
Definition
Monomers are the simple compounds from which polymers are made; other alkenes also polymerise
monomers. Other alkenes also undergo polymerisation.
-- NCERT Class 11 Chemistry, Ch. 9, p. 314Polymers are used for the manufacture of plastic bags, squeeze bottles, refrigerator dishes, toys, pipes, radio and T.V. cabinets etc. Polypropene is used for the manufacture of milk crates, plastic buckets and other moulded articles.
-- NCERT Class 11 Chemistry, Ch. 9, p. 314Though these materials have now become common, excessive use of polythene and polypropylene is a matter of great concern for all of us.
-- NCERT Class 11 Chemistry, Ch. 9, p. 314Alkene Polymerization: NEET previous year questions (PYQs) with answers
11 questions in Hydrocarbons
No question in our NEET 2020–2025 set targets this topic directly.
More in Hydrocarbons: 4 exam traps and mistakes · 1 formula · 2 question patterns from its other lessons.
Sources
NCERT refs: Class 11 Chemistry Chapter 9, p.313 | Class 11 Chemistry Chapter 9, p.314
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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