A hydrocarbon with the formula C_xH_y undergoes complete combustion as shown in the following equation: 2C_xH_y + 9O_2 → 6CO_2 + 6H_2O (a) What are the values of 'x' and 'y'? (b) Give the chemical (IUPAC) name of the hydrocarbon. (c) Draw its electron dot structure. (d) Name the alcohol which on heating with conc. H_2SO_4 will produce the above hydrocarbon C_xH_y. (e) Write a balanced chemical equation for the reaction of C_xH_y with hydrogen gas in presence of Nickel.

ScienceCBSE Class 10 · Sample paper, session 2025-26 · 5 marks · Case-based

CBSE Class 10 Science sample paper 2025-26: question 29(A)

From the official CBSE sample question paper for 2025-26, section B, question 29(A), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q29(A) · 5 marks · Case-basedCompetency-basedCarbon and its Compounds

A hydrocarbon with the formula $C_{x} H_{y}$ undergoes complete combustion as shown in the following equation:
$2 C_{x} H_{y} + 9 O_{2} \to 6 C O_{2} + 6 H_{2} O$

(a)
What are the values of 'x' and 'y'?
1 mark
(b)
Give the chemical (IUPAC) name of the hydrocarbon.
1 mark
(c)
Draw its electron dot structure.
1 mark
(d)
Name the alcohol which on heating with conc. H $_{2}$ SO $_{4}$ will produce the above hydrocarbon $C_{x} H_{y}$ .
1 mark
(e)
Write a balanced chemical equation for the reaction of $C_{x} H_{y}$ with hydrogen gas in presence of Nickel.
1 mark

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

Open one hint at a time and try again before the next.

Hint 1

Count C and H atoms on both sides of the equation.

Hint 2

Compare $C_{3} H_{6}$ with the general formulae of alkanes, alkenes and alkynes.

Hint 3

Removing water from an alcohol gives an alkene with the same number of carbons.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(a) Balance carbon: $2 x = 6 \Rightarrow x = 3$ . Balance hydrogen: $2 y = 6 \times 2 = 12 \Rightarrow y = 6$ . Check oxygen: $18 = 12 + 6$ ✓. So x = 3, y = 6 and the hydrocarbon is $C_{3} H_{6}$ .
1 mark
(b) $C_{3} H_{6}$ fits $C_{n} H_{2 n}$ , so it is an alkene: propene, $C H_{2} = C H - C H_{3}$ .
1 mark
(c) Electron dot structure: C1 and C2 share two pairs of electrons (double bond), and C2 and C3 share one pair. C1 shares one pair each with 2 H atoms, C2 with 1 H atom and C3 with 3 H atoms. Each carbon thus has 8 electrons around it and each H has 2.
1 mark
(d) Propanol (propan-1-ol) on heating with concentrated $H_{2} S O_{4}$ loses water (dehydration) to give propene: $C H_{3} C H_{2} C H_{2} O H co n c . H_{2} S O_{4}, h e a t C H_{3} C H = C H_{2} + H_{2} O$ .
1 mark
(e) Hydrogenation: $C H_{2} = C H - C H_{3} + H_{2} N i C H_{3} - C H_{2} - C H_{3}$ , i.e. $C_{3} H_{6} + H_{2} N i C_{3} H_{8}$ (propane).
1 mark
For visually impaired students: (a) x = 3, y = 6; (b) propene; (c) unsaturated hydrocarbon, as it contains a C=C double bond; (d) propanol; (e) $C_{3} H_{6} + H_{2} N i C_{3} H_{8}$ .

Answer: (a) x = 3, y = 6; (b) propene; (c) dot structure with C=C double bond; (d) propanol; (e) $C_{3} H_{6} + H_{2} N i C_{3} H_{8}$

Where the marks are

x = 3, y = 6 (1 mark)
Propene (1 mark)
Correct electron dot structure showing the double bond (1 mark)
Propanol (1 mark)
Balanced hydrogenation equation with Ni (1 mark)

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Chapter: Carbon and its Compounds (notes, formulas, where marks are lost and every past question)

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CBSE Class 10 Science sample paper 2025-26: question 29(A)

Hints

Step-by-step solution

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