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NCERT Class 10 Science Carbon and its Compounds: In-text questions (page 68) Solutions

Step-by-step working for every question of In-text questions (page 68) in NCERT's Class 10 Science book, chapter “Carbon and its Compounds”, written by Shishya. Each question is in our own short words; open the NCERT chapter for the exact wording and figures.

1. In-text (page 68) Q1

   How many structural isomers can you draw for pentane?

   1. Pentane has the molecular formula C₅H₁₂.
   2. The first isomer is n-pentane, which is a straight chain of five carbon atoms.
   3. The second isomer is isopentane (2-methylbutane), where a methyl group is attached to the second carbon of a butane chain.
   4. The third isomer is neopentane (2,2-dimethylpropane), where two methyl groups are attached to the central carbon of a propane chain.
   5. No other arrangements are possible without changing the connectivity or exceeding the valency of carbon.
   6. Thus, there are 3 structural isomers of pentane.

   Answer: 3 structural isomers

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2. In-text (page 68) Q2

   Which two properties of carbon lead to the huge number of carbon compounds?

   1. Carbon has four valence electrons, which allows it to form four covalent bonds with other atoms.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   2. This property is called tetravalency, and it enables carbon to bond with itself and other elements in many ways.
   3. Carbon atoms can link together to form long chains, branched structures, or rings, a property known as catenation.
   4. The combination of tetravalency and catenation allows carbon to form a vast variety of stable compounds.

   Answer: Tetravalency and catenation

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3. In-text (page 68) Q3

   What are the formula and electron-dot structure of cyclopentane?

   1. Cyclopentane is a cyclic alkane with five carbon atoms in a ring.
   2. The general formula for cycloalkanes is CnH2n, where n is the number of carbon atoms.
   3. Substituting n = 5 into the formula gives C5H10.
   4. Each carbon atom forms four covalent bonds to satisfy its tetravalency.
      why?

      In a covalent bond two atoms share a pair of electrons so that each gets a full outer shell. Carbon forms such bonds because gaining or losing four electrons would need too much energy.

   5. In the ring, each carbon is bonded to two other carbon atoms.
   6. Each carbon atom also forms two single bonds with hydrogen atoms.
   7. The electron-dot structure shows a pentagon of carbon atoms with two dots (electrons) shared between each adjacent pair of carbons.
   8. Two hydrogen atoms are attached to each carbon atom in the structure.

   Answer: C5H10

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4. In-text (page 68) Q4

   Describe the structures of (i) ethanoic acid (ii) bromopentane (iii) butanone (iv) hexanal. Are structural isomers possible for bromopentane?

   1. (i) Ethanoic acid has the formula CH₃COOH, with a methyl group attached to a carboxyl group.
   2. (ii) Bromopentane has the formula C₅H₁₁Br, where a bromine atom replaces one hydrogen in a pentane chain.
   3. (iii) Butanone has the formula CH₃COCH₂CH₃, featuring a ketone group at the second carbon.
   4. (iv) Hexanal has the formula CH₃(CH₂)₄CHO, with an aldehyde group at the end of a six-carbon chain.
   5. Yes, structural isomers are possible for bromopentane.
   6. The bromine atom can be attached to the 1st, 2nd, or 3rd carbon of the pentane chain.
   7. Moving the bromine gives 1-bromopentane, 2-bromopentane and 3-bromopentane; branched carbon chains (such as 1-bromo-2-methylbutane) give still more isomers.

   Answer: (i) CH₃COOH; (ii) C₅H₁₁Br, e.g. CH₃CH₂CH₂CH₂CH₂Br; (iii) CH₃COCH₂CH₃; (iv) CH₃(CH₂)₄CHO. Yes, bromopentane has several structural isomers (1-, 2- and 3-bromopentane, and branched ones).

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5. In-text (page 68) Q5

   Name the compounds drawn in the book: (i) CH₃–CH₂–Br (ii) a carbon atom joined to two hydrogens and double-bonded to oxygen (H–CHO) (iii) a six-carbon chain with a triple bond between the first two carbons.

   Uses a figure printed in the NCERT book — open the chapter above to see it.

   1. (i) The structure CH₃–CH₂–Br contains a two-carbon chain with a bromine atom attached to the terminal carbon.
   2. (i) The parent alkane for a two-carbon chain is ethane, and the bromine substituent makes it an alkyl halide.
   3. (i) Therefore, the compound is named bromoethane.
   4. (ii) The structure H–CHO represents a carbon atom bonded to one hydrogen and double-bonded to an oxygen atom (carbonyl group).
   5. (ii) This functional group is an aldehyde, and the simplest aldehyde with one carbon is derived from methane.
   6. (ii) Therefore, the compound is named methanal.
   7. (iii) The structure is a six-carbon chain with a triple bond between the first and second carbon atoms.
   8. (iii) The parent hydrocarbon for a six-carbon chain is hexane, and the presence of a triple bond changes the suffix to -yne.
   9. (iii) Since the triple bond starts at the first carbon, the position number is 1.
   10. (iii) Therefore, the compound is named hex-1-yne or 1-hexyne.

   Answer: (i) Bromoethane; (ii) Methanal; (iii) Hex-1-yne

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