1 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 $C_3H_4$ (propyne) and the 3rd member is $C_4H_6$ (butyne).

C3H4C4H6propynebutynealkynes

Marking Scheme

1Identification of the 2nd member as $C_3H_4$ : 0.5 mark
2Identification of the 3rd member as $C_4H_6$ : 0.5 mark

Hint

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

Quick Oral Answer

The first member of the alkyne series is ethyne ( $C_2H_2$ ). The subsequent members are propyne ( $C_3H_4$ ) and butyne ( $C_4H_6$ ), which differ by successive $-CH_2-$ units.

Analysis & Explanation

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

The first member of this series is ethyne ( $C_2H_2$ ), where $n = 2$ .
The second member is propyne ( $C_3H_4$ ), where $n = 3$ .
The third member is butyne ( $C_4H_6$ ), where $n = 4$ .

Common Mistakes

1Writing $C_2H_2$ as the second member by assuming $n=1$ is possible for alkynes.
2Confusing alkynes with alkenes ( $C_nH_{2n}$ ) or alkanes ( $C_nH_{2n+2}$ ).

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.

Spotted a mistake or something unclear?

Tell us — we fix reported answers fast.

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.