Mole Concept Gas Volume Stoichiometry Question JEE Main

Medium 10 Apr 2026
Nishant Kumar Gupta
Nishant Kumar Gupta
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Mole Concept Gas Volume Stoichiometry Question JEE Main 2026

Quick Summary

Question Type: Stoichiometry and gas volume statement-based question
Chapter: Mole Concept
Difficulty: ⭐⭐⭐ Medium
Time to Solve: 3-4 minutes
Key Formula: 1 mol gas occupies 22.4 L at STP; use stoichiometric mole ratios from the balanced equation before converting to gas volume
Correct Answer: (B) Only statement B is correct
Why: From 6 mol HCl, 3 mol H2 are formed. So 1 mol HCl gives 0.5 mol H2, which is 11.2 L at STP. The other statements are incorrect because solution volume cannot be compared directly like gas volume, and gas volume depends on temperature and pressure unless STP is specified.

The Question

JEE Main 2026 (Online) 22 January Morning Shift – Mole Concept

In the reaction,

2Al(s) + 6HCl(aq) → 2Al3+(aq) + 6Cl−(aq) + 3H2(g)

  • A) 12 L HCl(aq) is consumed for every 6 L H2(g) produced.
  • B) 11.2 L H2(g) at STP is produced for every mole of HCl consumed.
  • C) 33.6 L H2(g) is produced regardless of temperature and pressure for every mole of Al that reacts.
  • D) 67.2 L H2(g) at STP is produced for every mole of Al that reacts.

Quick Answer

Correct Answer: (B) Only statement B is correct

Reasoning: Use the balanced equation:

  • 6 mol HCl produce 3 mol H2
  • So 1 mol HCl produces 0.5 mol H2
  • At STP, 0.5 mol H2 occupies 11.2 L

Therefore, statement B is correct.

  • A is false: volume of HCl(aq) cannot be compared directly from stoichiometric coefficients unless concentration is specified
  • B is true: 1 mol HCl gives 0.5 mol H2 = 11.2 L at STP
  • C is false: 33.6 L is true only at STP, not regardless of temperature and pressure
  • D is false: 1 mol Al gives only 1.5 mol H2 = 33.6 L at STP, not 67.2 L

Video Solution

If you want the full explanation in a clear step-by-step teaching format, watch the video solution below:

Watch Full Video Solution on YouTube

Understanding the Concept

How to Solve Gas Volume Stoichiometry Questions

For such questions, the first step is to read the balanced equation carefully and extract the mole ratio between reactants and products.

After finding moles, convert only the gas amount into volume using the molar volume at STP:

  • 1 mol gas = 22.4 L at STP

A very common trap in JEE Main is mixing:

  • moles of reactants
  • volumes of gases
  • volumes of solutions

The Key Principle

  1. Use stoichiometric coefficients to find mole ratio
  2. Convert gas moles to volume only when STP is mentioned
  3. Do not compare solution volume directly unless concentration information is given

Detailed Step-by-Step Solution

Step 1: Read the Balanced Equation Properly

  • 2Al + 6HCl → 3H2
  • So 6 mol HCl produce 3 mol H2
  • Also, 2 mol Al produce 3 mol H2

Step 2: Check Statement B

  • 6 mol HCl produce 3 mol H2
  • 1 mol HCl produces 3/6 = 0.5 mol H2
  • At STP, 1 mol gas = 22.4 L
  • So 0.5 mol H2 = 11.2 L

So statement B is correct.

Step 3: Check Statement A

  • Stoichiometric coefficients compare moles, not arbitrary solution volumes
  • HCl is given as aqueous solution, not pure gas
  • Without concentration, liters of HCl(aq) cannot be directly related to liters of H2

So statement A is false.

Step 4: Check Statements C and D

Handwritten solution explaining gas volume stoichiometry for JEE Main 2026 Mole Concept question


Download Handwritten Solution

For 1 mol Al:

  • 2 mol Al produce 3 mol H2
  • So 1 mol Al produces 1.5 mol H2
  • At STP, volume = 1.5 × 22.4 = 33.6 L

Now check each statement:

  • C is false: 33.6 L is true only at STP, not at all temperatures and pressures
  • D is false: 67.2 L corresponds to 3 mol H2, not 1.5 mol H2

Step 5: Write Final Answer

Only statement B is correct.

Final Answer

(B) Only statement B is correct ✓

11.2 L of H2 at STP is produced for every mole of HCl consumed.

Why the Other Statements Are Wrong

  • Statement A: Incorrect because HCl(aq) volume depends on concentration, so it cannot be compared directly using stoichiometric coefficients.
  • Statement C: Incorrect because gas volume changes with temperature and pressure. 33.6 L is valid only at STP.
  • Statement D: Incorrect because 1 mol Al gives 1.5 mol H2, which is 33.6 L at STP, not 67.2 L.

Essential Formulas for This Topic

Primary Rules

  1. Molar Volume of Gas at STP:
    • 1 mol gas = 22.4 L at STP
  2. Stoichiometric Ratio Rule:
    • Coefficients in the balanced equation give mole ratios
    • Use these ratios before any volume conversion
  3. Solution Volume Caution:
    • Volume of solution is not directly proportional to mole ratio unless concentration is known

Important Notes

  • Gas volume comparison is valid only under specified conditions like STP
  • Liters of solution cannot be used directly without molarity or concentration
  • 1 mol Al gives 1.5 mol H2 in this reaction

Common Mistakes to Avoid

❌ Mistake 1: Comparing Liters of Solution and Gas Directly

Wrong Thinking: “The coefficient ratio directly gives liters of HCl(aq) and H2.”

Correct Approach: Coefficients compare moles. Solution volume needs concentration information.

❌ Mistake 2: Forgetting the STP Condition

Wrong Thinking: “If I know gas moles, volume is fixed in every condition.”

Correct Approach: Gas volume is fixed only when pressure and temperature are specified.

❌ Mistake 3: Wrong Ratio for Aluminum

Wrong Thinking: “1 mol Al gives 3 mol H2.”

Correct Approach: 2 mol Al give 3 mol H2, so 1 mol Al gives 1.5 mol H2.

Key Concept Summary

  1. 6 mol HCl give 3 mol H2: So 1 mol HCl gives 0.5 mol H2.
  2. 0.5 mol H2 occupies 11.2 L at STP: This makes statement B correct.
  3. 1 mol Al gives 1.5 mol H2: So 33.6 L at STP is correct for 1 mol Al.
  4. Gas volume depends on conditions: So statement C is false.
  5. Solution volume cannot be used directly: So statement A is false.

The Golden Rule for Gas Volume Questions

“First use mole ratio from the balanced equation, then convert gas moles into volume only under given conditions.”

Frequently Asked Questions

Q1: Why is statement B correct?

A: Because 1 mol HCl gives 0.5 mol H2, and 0.5 mol gas occupies 11.2 L at STP.

Q2: Why is statement A wrong?

A: Because HCl(aq) is a solution, and its volume cannot be related directly without knowing concentration.

Q3: How much H2 is formed from 1 mol Al?

A: 1.5 mol H2, which is 33.6 L at STP.

Q4: Why is statement C false?

A: Because gas volume changes with temperature and pressure, so 33.6 L is not valid under all conditions.

Q5: What is the correct answer?

A: Only statement B is correct.

Prerequisites to Solve This Question

  1. Balanced equations: Understanding stoichiometric coefficients
  2. Mole concept: Mole relationships from reactions
  3. Molar volume of gases: 22.4 L at STP
  4. Difference between solution and gas volume: Important for statement-based questions

After Solving This, You Can:

✅ Use stoichiometric ratios confidently
✅ Convert gas moles into volume at STP
✅ Avoid mixing solution volume and gas volume
✅ Solve statement-based Mole Concept questions faster

Study Tips for This Topic

For JEE Main:

  1. Always begin from mole ratio: Do not jump directly to volume comparison
  2. Check whether STP is given: Gas volume formulas depend on conditions
  3. Be careful with aqueous reactants: Solution volume needs concentration
  4. Verify each statement separately: JEE often mixes one correct statement with multiple traps

Common JEE Variants:

  • Gas volume at STP from balanced equations
  • Correct statement based stoichiometry questions
  • Mole to volume conversion of gases
  • Reaction stoichiometry involving metals and acids

Difficulty Rating & Exam Frequency

Difficulty Level: ⭐⭐⭐ (3/5) – Medium
JEE Main Frequency: High – Mole Concept and gas volume based statement questions are common
JEE Advanced Frequency: Medium – Often linked with redox or gas law concepts
Topic Importance: Very High – Stoichiometry and STP-based gas volume are foundational chemistry tools

Written by Nishant Kumar Gupta
Chemistry Educator with 12+ Years of Experience Teaching JEE Aspirants
Founder – PadhoLikhoJEE

Last Updated: April 2026
Question Source: JEE Main 2026 PYQ
Topic: Mole Concept – Gas Volume Stoichiometry

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