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MCQEasyJEE 2023Stoichiometry & Calculations

JEE Chemistry 2023 Question with Solution

When a hydrocarbon A undergoes combustion in the presence of air, it requires 9.59.5 equivalents of oxygen and produces 33 equivalents of water. What is the molecular formula of A?

  • A

    C8H6C_8H_6

  • B

    C9H9C_9H_9

  • C

    C6H6C_6H_6

  • D

    C9H6C_9H_6

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: A hydrocarbon has formula CxHyC_xH_y. It requires 9.59.5 equivalents of oxygen for complete combustion and produces 33 equivalents of water.

Find: The molecular formula of hydrocarbon A.

For complete combustion,

CxHy+O2→xCO2+y2H2OC_xH_y + O_2 \rightarrow xCO_2 + \frac{y}{2}H_2O

From the amount of water formed,

y2=3\frac{y}{2} = 3

So,

y=6y = 6

The oxygen required in combustion is

x+y4=9.5x + \frac{y}{4} = 9.5

Substituting y=6y = 6,

x+64=9.5x + \frac{6}{4} = 9.5 x+1.5=9.5x + 1.5 = 9.5 x=8x = 8

Therefore, the molecular formula of A is C8H6C_8H_6. The correct option is A.

The solution states B, but the worked steps clearly give C8H6C_8H_6, which matches option A.

Common mistakes

  • Using the water coefficient as y=3y = 3 instead of y2=3\frac{y}{2} = 3 is incorrect because one molecule of H2OH_2O contains two hydrogen atoms. Always equate water formed to y2\frac{y}{2} for CxHyC_xH_y.

  • Writing oxygen required as only xx ignores the oxygen needed to form water. In combustion of CxHyC_xH_y, total oxygen needed is x+y4x + \frac{y}{4}, not just the carbon contribution.

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