Class 11 Chemistry Numerical Worksheet Chapter 1 Some basic concepts of chemistry | Detailed Solutions & Marking Scheme

 Got it — I’ve reframed all 30 questions into slightly more elaborated, competency-based, student-friendly format while keeping them numerical-focused and beginner level 👇

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Class 11 Chemistry – Chapter 1: Some Basic Concepts of Chemistry

📄 Competency-Based Worksheet (30 Questions – Elaborated Version)

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🧠 Section A: Basic Understanding (Q1–Q6)

Q1. Calculate the molecular mass of carbon dioxide (CO₂) and sulphuric acid (H₂SO₄) using atomic masses (C = 12, O = 16, H = 1, S = 32).

Q2. Determine the number of moles present in 44 g of CO₂. Also identify what this mass represents in terms of molar mass.

Q3. Calculate the total number of molecules present in 18 g of water. Explain the steps involved.

Q4. A sample contains 1 mole of methane (CH₄). Find the total number of atoms present in the sample.

Q5. Calculate the total number of electrons present in one molecule of ammonia (NH₃). (Atomic numbers: N = 7, H = 1)

Q6. Determine the number of molecules present in 2 moles of oxygen gas (O₂).

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⚡ Section B: Mole & Particle Conversion (Q7–Q12)

Q7. Calculate the number of atoms present in 23 g of sodium (Na). (Atomic mass = 23)

Q8. A gas occupies 5.6 L at STP. Calculate the number of moles present in the gas sample.

Q9. Find the volume occupied by 0.5 moles of an ideal gas at STP.

Q10. How many moles of molecules are present in (6.022 \times 10^{22}) molecules of a substance?

Q11. Calculate the total number of molecules present in 11.2 L of a gas at STP.

Q12. Determine the number of moles present in 32 g of oxygen gas (O₂).

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🌡️ Section C: STP-Based Applications (Q13–Q18)

Q13. Calculate the number of molecules present in 22.4 L of nitrogen gas at STP.

Q14. Find the mass of oxygen gas that occupies 11.2 L at STP.

Q15. One mole of carbon dioxide gas is formed. Calculate the volume it will occupy at STP.

Q16. A gas occupies 44.8 L at STP. Calculate the number of moles present.

Q17. Determine the number of molecules present in 2.24 L of a gas at STP.

Q18. A sample contains 3 moles of a gas. Calculate the volume it occupies at STP.

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🧪 Section D: Concentration Terms (Q19–Q24)

Q19. A solution contains 2 moles of solute dissolved to make 1 litre of solution. Calculate its molarity.

Q20. Calculate the molality of a solution prepared by dissolving 1 mole of solute in 500 g of solvent.

Q21. Find the mole fraction of solute when 2 moles of solute are mixed with 8 moles of solvent.

Q22. A solution is prepared by dissolving 10 g of solute in 90 g of solvent. Calculate its percentage composition by mass (% w/w).

Q23. Calculate % (w/v) of a solution prepared by dissolving 5 g solute in 100 mL solution.

Q24. A mixture is prepared by mixing 20 mL ethanol with 80 mL water. Calculate the % (v/v) of ethanol.

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🔥 Section E: Competency-Based Thinking (Q25–Q30)

Q25. Calculate the number of moles present in 9 g of water. (H = 1, O = 16)

Q26. Determine the total number of atoms present in 11 g of CO₂.

Q27. Calculate the number of molecules present in 2.24 L of a gas at STP.

Q28. A solution is prepared by dissolving 2 moles of solute in 1 kg of solvent. Calculate its molality.

Q29. Find the mole fraction of solute when 3 moles of solute are mixed with 7 moles of solvent.

Q30. A solution contains 20 g of solute dissolved in 180 g of water. Calculate the percentage composition by mass (% w/w) of the solution.

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Perfect — now I’ll present the same answer key in fully detailed, step-by-step form, using simple plain maths format (easy to copy, clean powers, no fancy symbols) exactly as students should write in exams 👇

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Class 11 Chemistry – Chapter 1

📄 Detailed Step-by-Step Answer Key (Exam Writing Format)

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🧠 Section A (1 mark each)

Q1. Molecular Mass

CO2 = 12 + (2 × 16)
= 12 + 32
= 44 g/mol

H2SO4 = (2 × 1) + 32 + (4 × 16)
= 2 + 32 + 64
= 98 g/mol

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Q2. Number of moles in 44 g CO2

Moles = Mass / Molar mass
= 44 / 44
= 1 mole

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Q3. Number of molecules in 18 g H2O

Moles = 18 / 18 = 1 mole

Number of molecules = moles × NA
= 1 × 6.022 × 10^23
= 6.022 × 10^23 molecules

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Q4. Number of atoms in 1 mole CH4

Atoms in one molecule = 1 + 4 = 5

Total atoms = 5 × 6.022 × 10^23
= 3.011 × 10^24 atoms

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Q5. Number of electrons in NH3

N = 7 electrons
H = 1 electron

Total electrons = 7 + (3 × 1)
= 10 electrons

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Q6. Molecules in 2 moles O2

= 2 × 6.022 × 10^23
= 1.204 × 10^24 molecules

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⚡ Section B (1 mark each)

Q7. Atoms in 23 g Na

Moles = 23 / 23 = 1

Atoms = 1 × 6.022 × 10^23
= 6.022 × 10^23 atoms

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Q8. Moles in 5.6 L gas (STP)

Moles = Volume / 22.4
= 5.6 / 22.4
= 0.25 mole

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Q9. Volume of 0.5 mole gas (STP)

Volume = moles × 22.4
= 0.5 × 22.4
= 11.2 L

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Q10. Moles from molecules

Moles = (6.022 × 10^22) / (6.022 × 10^23)
= 10^22 / 10^23
= 0.1 mole

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Q11. Molecules in 11.2 L gas

Moles = 11.2 / 22.4 = 0.5

Molecules = 0.5 × 6.022 × 10^23
= 3.011 × 10^23 molecules

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Q12. Moles in 32 g O2

Moles = 32 / 32
= 1 mole

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🌡️ Section C (1 mark each)

Q13. Molecules in 22.4 L N2

Moles = 1

Molecules = 1 × 6.022 × 10^23
= 6.022 × 10^23 molecules

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Q14. Mass of O2 in 11.2 L

Moles = 11.2 / 22.4 = 0.5

Mass = moles × molar mass
= 0.5 × 32
= 16 g

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Q15. Volume of 1 mole CO2

Volume = 1 × 22.4
= 22.4 L

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Q16. Moles in 44.8 L gas

= 44.8 / 22.4
= 2 moles

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Q17. Molecules in 2.24 L gas

Moles = 2.24 / 22.4 = 0.1

Molecules = 0.1 × 6.022 × 10^23
= 6.022 × 10^22 molecules

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Q18. Volume of 3 moles gas

= 3 × 22.4
= 67.2 L

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🧪 Section D (2 marks each)

Q19. Molarity

M = moles / volume (in L)
= 2 / 1
= 2 M

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Q20. Molality

Molality = moles / kg solvent
= 1 / 0.5
= 2 m

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Q21. Mole fraction

= moles of solute / total moles
= 2 / (2 + 8)
= 2 / 10
= 0.2

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Q22. % (w/w)

Total mass = 10 + 90 = 100 g

% (w/w) = (10 / 100) × 100
= 10%

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Q23. % (w/v)

% (w/v) = (5 / 100) × 100
= 5%

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Q24. % (v/v)

Total volume = 20 + 80 = 100 mL

% (v/v) = (20 / 100) × 100
= 20%

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🔥 Section E (2 marks each)

Q25. Moles in 9 g H2O

Moles = 9 / 18
= 0.5 mole

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Q26. Atoms in 11 g CO2

Moles = 11 / 44 = 0.25

Atoms per molecule = 3

Total atoms = 0.25 × 3 × 6.022 × 10^23
= 0.75 × 6.022 × 10^23
= 4.516 × 10^23 atoms

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Q27. Molecules in 2.24 L gas

Moles = 2.24 / 22.4 = 0.1

Molecules = 0.1 × 6.022 × 10^23
= 6.022 × 10^22 molecules

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Q28. Molality

= 2 / 1
= 2 m

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Q29. Mole fraction

= 3 / (3 + 7)
= 3 / 10
= 0.3

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Q30. % (w/w)

Total mass = 20 + 180 = 200 g

% (w/w) = (20 / 200) × 100
= 10%

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🔥 Common Mistakes Sheet (Mole Concept)

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❌ 1. Confusing Mass with Moles

Mistake: Students directly use given mass as moles
Wrong: 44 g CO₂ = 44 moles
Correct:
Moles = mass / molar mass = 44 / 44 = 1 mole

👉 Tip: Always divide by molar mass

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❌ 2. Forgetting Units in STP Questions

Mistake: Using 22.4 blindly even when not at STP
👉 Tip:

• Use 22.4 L only at STP

• Otherwise use formula PV = nRT

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❌ 3. Mixing Atoms and Molecules

Mistake:
1 mole CH₄ = 6.022 × 10^23 atoms ❌

Correct:
1 molecule CH₄ = 5 atoms
Total atoms = 5 × 6.022 × 10^23

👉 Tip: Always count atoms per molecule

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❌ 4. Wrong Use of Avogadro Number

Mistake: Directly multiplying without converting to moles
👉 Correct Approach:
Step 1: Convert to moles
Step 2: Multiply by 6.022 × 10^23

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❌ 5. Ignoring Total Mass in % (w/w)

Mistake: % = (solute / solvent) × 100 ❌

Correct:
% (w/w) = (solute / solution) × 100

👉 Always add solute + solvent

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❌ 6. Confusion Between Molarity and Molality

Concept	Common Mistake	Correct
Molarity	using kg	uses litre
Molality	using litre	uses kg

👉 Tip:

• Molarity → volume

• Molality → mass

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❌ 7. Forgetting Mole Fraction Formula

Mistake: Taking only solute moles

Correct:
Mole fraction = moles of component / total moles

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❌ 8. Power Mistakes (10^23 Errors)

Mistake:
6.022 × 10^22 / 6.022 × 10^23 = 1 ❌

Correct:
= 10^22 / 10^23 = 10^-1 = 0.1

👉 Tip: Subtract powers carefully

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❌ 9. Not Identifying Given Quantity Properly

Students confuse whether given is:

• mass

• volume

• molecules

👉 Golden Rule:
👉 First identify → then apply formula

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❌ 10. Skipping Units in Final Answer

👉 Always write:

• mole

• L

• g

• molecules

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💡 Golden Shortcut Line (Teach This!)

👉 Mass → Moles → Particles → Atoms
(Always move step-by-step, never jump)

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⚡ Oral Viva Questions (Rapid-Fire Round)

Use this in class for speed + confidence + concept clarity

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🧠 Basic (Warm-Up)

1. What is 1 mole?

2. Value of Avogadro number?

3. Molar volume at STP?

4. Define molar mass

5. Unit of molarity?

6. Unit of molality?

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⚡ Concept Check

7. Which is bigger: 1 mole atoms or 1 mole molecules?

8. 22.4 L is valid at which condition?

9. What is mole fraction range?

10. Does molarity depend on temperature?

11. Does molality depend on temperature?

12. Which is better for temperature change: M or m?

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🔥 Numerical Quick Shots

13. Moles in 22.4 L gas?

14. Molecules in 1 mole?

15. Moles in 44 g CO₂?

16. Volume of 2 moles gas at STP?

17. Atoms in 1 mole O₂?

18. Moles in 11.2 L gas?

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🚀 Application Level

19. Why is molality preferred over molarity?

20. Why is 1 mole same for all substances?

21. Why do gases follow 22.4 L rule?

22. What happens to molarity when temperature increases?

23. Why do we use mole concept in chemistry?

24. Difference between atom and molecule in counting?

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💥 Challenge Round (Top Students)

25. Number of atoms in 1 mole CH₄?

26. Mole fraction if equal moles mixed?

27. % (w/w) if solute = solvent?

28. Moles in 6.022 × 10^22 molecules?

29. Volume of 0.25 mole gas at STP?

30. Which has more atoms: 1 mole H₂ or 1 mole He?

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