JEE Main · 2023 · 31 Jan 2023 · Shift 2 · Some Basic Concepts of Chemistry

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When a hydrocarbon A undergoes complete combustion it requires 11 equivalents of oxygen and produces 4 equivalents of water. What is the molecular formula of A ?

(A) $C_{9} H_{8}$ ✓ Correct
(B) $C_{11} H_{4}$
(C) $C_{5} H_{8}$
(D) $C_{11} H_{8}$

Answer: (A)

Solution

For complete combustion of a hydrocarbon

C_{x} H_{y}

the equation is

C_{x} H_{y} + (x + y/4) O_{2} \to x C O_{2} + (y/2) H_{2} O

. Four equivalents of water give

y /2 = 4

, so

y = 8

, and eleven equivalents of oxygen give

x + y /4 = 11

, so

x + 2 = 11

and

x = 9

. The molecular formula of A is

C_{9} H_{8}

JEE Main · 2023 · 31 Jan 2023 · Shift 2 · Single correct option

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