1 markVery Short AnswerSection A

Write the molecular formula of the $2^{nd}$ and $3^{rd}$ member of the homologous series whose first member is ethene.

Chemical Reactions and Equations

Homologous Series

Official Answer

The molecular formula of the $2^{nd}$ member is $C_3H_6$ (Propene) and the $3^{rd}$ member is $C_4H_8$ (Butene).

C_3H_6

C_4H_8

PropeneButeneAlkenes

Marking Scheme

10.5 marks for $C_3H_6$
20.5 marks for $C_4H_8$

Hint

The general formula for alkenes is $C_nH_{2n}$ . Start with $n=3$ for the second member.

Quick Oral Answer

The second and third members of the alkene series are propene ( $C_3H_6$ ) and butene ( $C_4H_8$ ). They follow the general formula $C_nH_{2n}$ .

Analysis & Explanation

The homologous series starting with ethene ( $C_2H_4$ ) is the alkene series, which follows the general formula $C_nH_{2n}$ .

The first member is ethene ( $n=2$ ): $C_2H_4$ .
The second member is propene ( $n=3$ ): $C_3H_6$ .
The third member is butene ( $n=4$ ): $C_4H_8$ .

As per the NCERT text (Section 1.1.1), chemical formulae are used to represent substances concisely. In a homologous series, each successive member differs by a $-CH_2-$ unit.

Common Mistakes

1Starting the count from $n=1$ (methene does not exist).
2Confusing alkenes with alkanes ( $C_nH_{2n+2}$ ).
3Writing names instead of molecular formulae.

Interesting Facts

Ethene is a natural plant hormone used to ripen fruits, a process involving complex chemical changes similar to those mentioned in the chapter introduction.

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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 "Chemical Reactions and Equations" 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.