What is the answer to the Stoichiometry and Gas Laws NEET 2026 question?
Option B is correct: the final mixture contains 0.8 dm³ CO and 0.6 dm³ CO₂. This stoichiometry and gas laws NEET 2026 question turns on one distinction: the volume of CO₂ consumed is not the volume of CO formed.
A 1 dm³ sample of CO₂ passes over heated coke. After the reaction ends, the gaseous mixture occupies 1.4 dm³ at STP. Find the separate volumes of CO and CO₂ at STP.
The chapter is Some Basic Concepts in Chemistry. The supplied question bank rates this single-correct question medium, with an expected solving time of 90 seconds, not a compulsory examination time limit.
Why does one volume of CO₂ produce two volumes of CO?
One mole of CO₂ consumed produces two moles of CO. At the same temperature and pressure, gas volumes follow that ratio: one volume of CO₂ consumed produces two volumes of CO.
The balanced reaction is:
The ideal-gas equation gives:
At a common temperature and pressure, gas volume is proportional to the amount in moles. Therefore:
Use the common-condition volume basis implicit in the official method. Do not compare a gas volume inside the hot reaction zone directly with a collected STP volume.
Coke is solid carbon. It participates in the reaction but contributes no term to the gaseous-mixture volume. Conversion through the STP molar volume is unnecessary because the common factor cancels:
How should you label the CO₂ consumed, CO₂ remaining and CO formed?
Define the unknown as the volume of CO₂ consumed, exactly as in the official solution. It is neither the residual CO₂ volume nor the CO volume.
Use this labelled gas-volume ledger, with every entry on the same temperature-and-pressure basis:
- Initial:
- Change:
- Final:
The subtraction tracks reactant consumption. The factor of two tracks product formation from the balanced equation.
“The reaction has ended” does not mean all the initial CO₂ was consumed. Complete consumption would produce 2 dm³ of CO, with no CO₂ left, contradicting the stated final volume of 1.4 dm³. The measurement requires residual CO₂; the question does not supply a reason why conversion stopped.
How do you solve the volume equation and verify option B?
Add only the two final gas volumes: residual CO₂ and newly formed CO. Their sum must equal the measured 1.4 dm³.
Using numerical volumes in dm³:
Calculate each labelled volume separately:
Thus, option B contains 0.8 dm³ CO and 0.6 dm³ CO₂. Verify it in two ways, because checking the sum alone does not test the species assignments.
- Total-volume check:
- Reaction-ratio check:
The shortcut follows from this accounting: each reacting volume of CO₂ is replaced by twice that volume of CO. The net volume increase equals the CO₂ volume consumed. Here, the increase is 0.4 dm³, matching the full calculation.
Why is option C wrong even though its volumes add to 1.4 dm³?
Option C satisfies the total-volume condition but violates the reaction ratio. Both B and C total 1.4 dm³, so addition cannot distinguish them.
After finding the consumed volume, swapping the species labels generates C:
This mistake applies the product coefficient to the residual reactant. Test C directly: 0.8 dm³ of residual CO₂ would imply:
That consumption would form:
C lists 0.6 dm³ CO, so it fails. Write the species names beside the remaining-reactant and product expressions before substituting numbers.
Can you solve three related gas-volume questions?
Use the same labelled ledger for all three problems below. These are author-created practice questions, not additional verified NEET PYQs, from Some Basic Concepts in Chemistry.
Every question uses this reaction, and all compared gas volumes are measured at the same temperature and pressure:
In each solution, the unknown is the numerical volume of CO₂ consumed in dm³.
What is the composition when 2.0 dm³ CO₂ gives 2.6 dm³ of mixture?
The mixture contains 1.2 dm³ CO and 1.4 dm³ CO₂ after passage over hot coke. Subtract consumption from the initial reactant volume, then add twice that consumption as product:
What remains if all of an initial 1.0 dm³ CO₂ reacts?
The final gas is 2.0 dm³ of CO only. Unlike the original question, this question explicitly states that all the CO₂ reacts.
What remains when 0.5 dm³ CO₂ produces 0.6 dm³ CO?
The residual CO₂ is 0.2 dm³, and the total gas volume is 0.8 dm³. Here, start with the given product volume to find consumption:
For your next problem, write initial gas volumes, subtract reactant consumption, add products using balanced coefficients, and only then sum the final gases.
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Related on NEET JEEnius AI: Free Fall Under Gravity NEET 2026: Falling-Ruler Solution.
Frequently asked questions
What is the answer to the CO₂ and heated coke gas-volume question?
Option B is correct: the final mixture contains 0.8 dm³ CO and 0.6 dm³ CO₂ at STP. On a common temperature-and-pressure basis, let x dm³ of the initial 1 dm³ CO₂ react; the final volume is (1 − x) + 2x = 1.4 dm³. Hence x = 0.4, giving 0.8 dm³ CO formed and 0.6 dm³ CO₂ remaining.
Why does one volume of CO₂ produce two volumes of CO?
The balanced reaction is CO₂(g) + C(s) → 2CO(g), so one mole of CO₂ produces two moles of CO. At the same temperature and pressure, gas volume is proportional to moles, giving the same 1:2 volume ratio. Solid carbon does not contribute to the gaseous-mixture volume.
Why is option C wrong if its gas volumes add to 1.4 dm³?
Option C gives 0.8 dm³ residual CO₂, which means only 0.2 dm³ of the initial CO₂ was consumed. That consumption would produce 0.4 dm³ CO, not the 0.6 dm³ listed in C. The option passes the total-volume check but fails the reaction-ratio check.
Do I need to use 22.4 L/mol to solve this gas-volume question?
No, conversion through the STP molar volume is unnecessary because the common factor cancels. Use the balanced equation to compare gas volumes directly, provided every compared volume is on the same temperature-and-pressure basis. Do not compare a volume inside the hot reaction zone directly with a collected STP volume.