Q.110 Chemistry ⭐ 360° Option Elimination Active

NEET 2020 Chemistry Question 110 Solution

Topic: States of Matter · Official NTA Booklet Code E1

Question

A mixture of N₂ and Ar gases in a cylinder contains 7 g of N₂ and 8 g of Ar. If the total pressure of the mixture of the gases in the cylinder is 27 bar, the partial pressure of N₂ is: [Use atomic masses (in g mol⁻¹): N = 14, Ar = 40]

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Verified Against NTA Final Answer Key

Option (3) : 15 bar

📚 States of Matter Easy

📋 Given Parameters & Problem Setup

Mass of N2 7 g (Molar mass of N2 = 28 g mol^-1)

Mass of Ar 8 g (Atomic mass of Ar = 40 g mol^-1)

Total Pressure (P_total) 27 bar

Question Objective Calculate the partial pressure of N2 (p_N2)

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The 30-Second Exam Hall Shortcut: ⚡ "n(N2) = 7/28 = 0.25 mol; n(Ar) = 8/40 = 0.20 mol; p(N2) = (0.25/0.45) × 27 = 15 bar!"

⏱️ 25 seconds

Calculate moles: $n_{N_2} = \frac{7}{28} = 0.25\text{ mol}$ , and $n_{Ar} = \frac{8}{40} = 0.20\text{ mol}$ . Total moles = $0.25 + 0.20 = 0.45\text{ mol}$ . Mole fraction of $N_2$ = $\frac{0.25}{0.45} = \frac{5}{9}$ . By Dalton's Law: $p_{N_2} = x_{N_2} \times P_{\text{total}} = \frac{5}{9} \times 27\text{ bar} = 5 \times 3 = 15\text{ bar}$ ! Mark Option (3) in 10 seconds.

1 Moles of N2 = 7 g / 28 g mol^-1 = 0.25 mol (or 1/4 mol).

2 Moles of Ar = 8 g / 40 g mol^-1 = 0.20 mol (or 1/5 mol).

3 Total moles = 0.25 + 0.20 = 0.45 mol.

4 Mole fraction of N2: x_N2 = 0.25 / 0.45 = 25/45 = 5/9.

5 Partial pressure: p_N2 = x_N2 × P_total = (5/9) × 27 bar = 15 bar.

6 Select Option (3).

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Why You Can 100% Trust This Shortcut: NCERT Chemistry · States of Matter (Dalton's Law of Partial Pressures)

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Calculate Moles of Each Gas

Molar mass of diatomic nitrogen ( $\text{N}_2$ ) = $2 \times 14 = 28\text{ g mol}^{-1}$ . Molar mass of noble gas argon ( $\text{Ar}$ ) = $40\text{ g mol}^{-1}$ . $n_{\text{N}_2} = \frac{7\text{ g}}{28\text{ g mol}^{-1}} = 0.25\text{ mol}$ $n_{\text{Ar}} = \frac{8\text{ g}}{40\text{ g mol}^{-1}} = 0.20\text{ mol}$

Step 2

Calculate Total Number of Moles

$n_{\text{total}} = n_{\text{N}_2} + n_{\text{Ar}} = 0.25 + 0.20 = 0.45\text{ mol}$

Step 3

Calculate Mole Fraction of Nitrogen

$x_{\text{N}_2} = \frac{n_{\text{N}_2}}{n_{\text{total}}} = \frac{0.25}{0.45} = \frac{25}{45} = \frac{5}{9}$

Step 4

Apply Dalton's Law of Partial Pressures

According to Dalton's law, the partial pressure of a gas in a mixture equals its mole fraction multiplied by the total pressure: $p_{\text{N}_2} = x_{\text{N}_2} \cdot P_{\text{total}}$ $p_{\text{N}_2} = \left(\frac{5}{9}\right) \times 27\text{ bar} = 5 \times 3 = 15\text{ bar}$ (Notice also that for argon, $p_{\text{Ar}} = \frac{4}{9} \times 27 = 12\text{ bar}$ , and $15 + 12 = 27\text{ bar}$ ).

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360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; arithmetic error in mole fraction.

⚠️

Student Fallacy (Why candidates pick this trap): Inverting fractions or dividing total pressure by 3.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; 12 bar is the partial pressure of ARGON ( $p_{\text{Ar}} = \frac{4}{9} \times 27 = 12\text{ bar}$ ), not nitrogen.

⚠️

Student Fallacy (Why candidates pick this trap): Accidentally calculating the partial pressure of the other gas (Argon).

3 C

✓ 100% Correct

Verdict & Scientific Reason:

Factually and mathematically exact; $p_{\text{N}_2} = \frac{5}{9} \times 27 = 15\text{ bar}$ .

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Why this is airtight: None; clean Dalton's law calculation.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect; overestimation of nitrogen fraction.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming nitrogen accounts for 2/3 of total pressure.

⚠️

NTA Trap Alert: Molar Mass of Nitrogen: N vs N2

Nitrogen gas is DIATOMIC ( $\text{N}_2$ ), so its molar mass is $28\text{ g/mol}$ , NOT $14\text{ g/mol}$ ! Using 14 gives $n = 0.5\text{ mol}$ , which leads to wrong options.

🛡️ How to avoid the -1 mark: Remember: Gas in cylinder is

\text{N}_2

-> Molar mass = 28 g/mol!

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Core NEET Syllabus Standard

Chemistry · States of Matter Standard Benchmark

Core Scientific Principle Tested:

Syllabus Focus: Dalton's Law of Partial Pressures: Partial Pressure of N2 in N2-Ar Mixture

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2020 Chemistry Question 110 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "n(N2) = 7/28 = 0.25 mol; n(Ar) = 8/40 = 0.20 mol; p(N2) = (0.25/0.45) × 27 = 15 bar!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Calculate Moles of Each Gas

Calculate Total Number of Moles

Calculate Mole Fraction of Nitrogen

Apply Dalton's Law of Partial Pressures

360° Option Elimination Engine Key Feature

NTA Trap Alert: Molar Mass of Nitrogen: N vs N2

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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