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When $1 \text{ dm}^3$ of $CO_2$ gas is passed over hot coke, the volume of gaseous mixture after complete reaction at STP becomes $1.4 \text{ dm}^3$. The composition of the gaseous mixture at STP is :

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NEET UG Re-Exam 2026 Question Paper (21-Jun-2026)
The correct answer is

$0.8 \text{ dm}^3$ of CO, $0.6 \text{ dm}^3$ of $CO_2$

Reaction Analysis: CO2 with Hot Coke

The reaction between carbon dioxide ($CO_2$) gas and hot coke (carbon, $C$) produces carbon monoxide (CO) gas. The balanced chemical equation is:

$CO_2(g) + C(s) \rightarrow 2CO(g)$

This equation shows that 1 volume of $CO_2$ reacts to produce 2 volumes of CO. This results in a net increase in the volume of gaseous products compared to the reactant consumed.

Volume Change Calculation

Let the initial volume of $CO_2$ gas be $V_{CO_2, initial} = 1 \text{ dm}^3$. Let $x$ represent the volume of $CO_2$ that reacts completely.

  • Volume of $CO_2$ consumed = $x \text{ dm}^3$.
  • Volume of CO produced = $2x \text{ dm}^3$ (based on the stoichiometry).
  • Volume of $CO_2$ remaining unreacted = $V_{CO_2, initial} - x = (1 - x) \text{ dm}^3$.

The total final volume of the gaseous mixture ($V_{final}$) is the sum of the unreacted $CO_2$ and the produced CO:

$V_{final} = (\text{Volume of } CO_2 \text{ remaining}) + (\text{Volume of } CO \text{ produced})$

$V_{final} = (1 - x) + 2x$

$V_{final} = 1 + x$

The problem states that the final volume after the complete reaction is $1.4 \text{ dm}^3$. Therefore:

$1 + x = 1.4 \text{ dm}^3$

Solving for $x$ gives:

$x = 1.4 - 1 = 0.4 \text{ dm}^3$

Final Gas Composition

Now, we calculate the volume of each gas present in the final mixture:

  • Volume of CO produced = $2x = 2 \times 0.4 = 0.8 \text{ dm}^3$.
  • Volume of $CO_2$ remaining = $1 - x = 1 - 0.4 = 0.6 \text{ dm}^3$.

Thus, the composition of the gaseous mixture at STP is $0.8 \text{ dm}^3$ of CO and $0.6 \text{ dm}^3$ of $CO_2$.

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