Question 11 markVery Short AnswerSection A

Write the molecular formula of the 2nd and 3rd member of the homologous series where the first member is ethyne.

Carbon and its Compounds
Homologous Series

Official answer

The molecular formula of the 2nd member is C3H4C_3H_4 (propyne) and the 3rd member is C4H6C_4H_6 (butyne).

Answer keywords:C3H4C4H6propynebutynealkynes

Marking scheme

  1. 1.Identification of the 2nd member as C3H4C_3H_4: 0.5 mark
  2. 2.Identification of the 3rd member as C4H6C_4H_6: 0.5 mark

Hint

Recall the general formula for alkynes, CnH2n−2C_nH_{2n-2}, where the first member starts with n=2n=2 (ethyne).

Quick oral answer

The first member of the alkyne series is ethyne (C2H2C_2H_2). The subsequent members are propyne (C3H4C_3H_4) and butyne (C4H6C_4H_6), which differ by successive −CH2−-CH_2- units.

Common mistakes

  1. 1.Writing C2H2C_2H_2 as the second member by assuming n=1n=1 is possible for alkynes.
  2. 2.Confusing alkynes with alkenes (CnH2nC_nH_{2n}) or alkanes (CnH2n+2C_nH_{2n+2}).

Analysis and explanation

The homologous series of alkynes has the general formula CnH2n−2C_nH_{2n-2}.

  • The first member of this series is ethyne (C2H2C_2H_2), where n=2n = 2.
  • The second member is propyne (C3H4C_3H_4), where n=3n = 3.
  • The third member is butyne (C4H6C_4H_6), where n=4n = 4.

Interesting facts

Alkynes contain at least one carbon-carbon triple bond. Because a triple bond requires two carbon atoms, the simplest alkyne (ethyne) must have at least two carbon atoms, meaning there is no single-carbon alkyne.

Question details

Q1 of 33 questions

Board
CBSE
Class
Class 10
Subject
Science
Year
2017
Section
Section A
Marks
1
Type
Very Short Answer
Chapter
Carbon and its Compounds
Topic
Homologous Series
Bloom's level
Applying
Syllabus topic
Homologous series, nomenclature of carbon compounds containing functional groups

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Frequently asked questions

How many marks does this question carry in CBSE Class 10 Science 2017?

This question carries 1 mark in the CBSE Class 10 Science 2017 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Homologous Series" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2017 paper?

This is a Very Short Answer question from Section A in the CBSE Class 10 Science 2017 paper.