1 markVery Short AnswerSection A

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

Chemical Reactions and Equations

Homologous Series of Alkanes

Official Answer

The molecular formula of the second member of the alkane homologous series (ethane) is $C_2H_6$ , and the molecular formula of the third member (propane) is $C_3H_8$ .

C2H6C3H8ethanepropane

Marking Scheme

10.5 marks for correctly identifying and writing the molecular formula of the 2nd member ( $C_2H_6$ ).
20.5 marks for correctly identifying and writing the molecular formula of the 3rd member ( $C_3H_8$ ).

Hint

Recall the general formula for alkanes, which is $C_nH_{2n+2}$ , where $n$ represents the number of carbon atoms. Methane has $n = 1$ . Find the formulas for $n = 2$ and $n = 3$ .

Quick Oral Answer

The second member is ethane with the formula $C_2H_6$ , and the third member is propane with the formula $C_3H_8$ . They belong to the alkane family and differ by a $-CH_2-$ group.

Analysis & Explanation

A homologous series is a family of organic compounds that share the same functional group and general chemical formula, with successive members differing by a $-CH_2-$ unit. For alkanes, the general formula is $C_nH_{2n+2}$ . Methane ( $CH_4$ ) is the first member ( $n=1$ ). Adding a $-CH_2-$ unit gives the second member, ethane ( $C_2H_6$ ), and adding another unit gives the third member, propane ( $C_3H_8$ ).

Common Mistakes

1Writing structural formulas instead of molecular formulas.
2Confusing alkanes with alkenes or alkynes, leading to incorrect formulas like $C_2H_4$ or $C_3H_6$ .

Interesting Facts

Ethane and propane are major components of natural gas and liquefied petroleum gas (LPG), which are widely used as clean-burning fuels.

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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 of Alkanes" 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.