Mole Concept and Avogadros Number - UNSOLVED PRACTICE SET

Q7. Define a mole. What is the significance of Avogadro's number in chemistry?

Q8. Calculate the number of moles in: (i) 12 g of oxygen gas (O₂), (ii) 5.6 g of nitrogen gas (N₂), (iii) 88 g of carbon dioxide (CO₂).

Q9. Your teacher asks you to explain the mole concept to a Class 9 student. How would you explain it using a simple analogy (like a dozen eggs or a score of bananas)?

Q10. Calculate the number of atoms in: (i) 0.5 mole of carbon, (ii) 2 moles of aluminium, (iii) 1 mole of sulphur (S₈).

Q11. What is the relationship between the mole, molar mass, and Avogadro's number? Write the formula connecting these three quantities.

Q12. Calculate the mass of: (i) 0.25 mole of NaOH, (ii) 3 moles of H₂SO₄, (iii) 0.1 mole of CaCO₃.

Q13. Explain the mole concept in detail. What is Avogadro's number, and why is it called a 'counting unit' for chemists? Describe how the mole connects the microscopic world of atoms to the macroscopic world of grams and kilograms. Calculate the number of molecules in 9 g of water.

Q14. A student is confused about why chemists use the mole instead of counting atoms directly. Help the student by:

(a) Explaining why counting individual atoms is impractical.

(b) Describing how the mole acts as a bridge between the atomic and macroscopic scales.

(c) Calculating how many atoms are present in 4 g of helium.

(d) Comparing the number of atoms in 1 g of hydrogen and 1 g of carbon. Which contains more atoms, and why?

Q15. "The mole concept is one of the most powerful tools in chemistry, used everywhere from cooking to space exploration." Discuss this statement by giving three real-life examples: (i) how it helps pharmacists prepare medicines in correct doses, (ii) how it helps environmental scientists measure pollutants, and (iii) how it helps NASA scientists calculate fuel requirements for rockets.

Q16. A chemistry laboratory has the following samples:

Sample A: 11.2 L of CO₂ gas at STP

Sample B: 22 g of CO₂

Sample C: 3.011 × 10²³ molecules of CO₂

(a) Calculate the number of moles in each sample.

(b) Which sample contains the maximum number of molecules?

(c) Calculate the total number of oxygen atoms in Sample B.

(d) If all three samples are combined, what is the total mass of CO₂?

Q17. A farmer needs to apply nitrogen fertilizer to his field. He has two options:

Option 1: Urea (CO(NH₂)₂) — costs ₹50 per kg

Option 2: Ammonium nitrate (NH₄NO₃) — costs ₹40 per kg

(a) Calculate the molecular mass of urea and ammonium nitrate.

(b) Calculate the percentage of nitrogen in each fertilizer.

(c) If the farmer needs 50 kg of nitrogen for his field, how many kilograms of each fertilizer does he need to buy?

(d) Which option is more economical for the farmer? Show your calculations.

Q18. A soft drink manufacturer produces carbonated water by dissolving CO₂ in water. Each bottle contains 500 mL of solution with 2 g of dissolved CO₂.

(a) Calculate the number of moles of CO₂ dissolved in one bottle.

(b) Calculate the number of molecules of CO₂ in one bottle.

(c) If the manufacturer produces 10,000 bottles per day, calculate the total mass of CO₂ used daily.

(d) Calculate the total number of CO₂ molecules used daily.

(e) Why is the mole concept useful for the manufacturer in planning production?