Mole Concept and Molar Mass - UNSOLVED PRACTICE SET

Q7. Define a mole. Why is it called chemists' counting unit?

Q8. What is molar mass? How is it different from molecular mass?

Q9. Calculate the number of moles in 88 g of CO₂.

Q10. What is Avogadro's constant? State its numerical value with units.

Q11. Your father buys a 1 kg LPG cylinder for cooking. LPG mainly contains propane (C₃H₈). If you know the molar mass of propane, explain how you would calculate how many moles of propane are in the cylinder. Why is this calculation useful for the gas company?

Q12. Why is the mole concept more useful than counting individual atoms or molecules in chemical calculations?

Q13. Explain the mole concept in detail. Define:

(a) Gram atomic mass

(b) Gram molecular mass

(c) Molar volume of a gas at STP

Derive the relationship between number of moles, mass, and molar mass. Calculate the number of molecules in 9 g of water.

Q14. Describe the significance of Avogadro's constant in chemistry. How does it bridge the microscopic world of atoms and the macroscopic world of grams and litres? Calculate:

(a) The mass of 3.011 × 10²³ atoms of carbon

(b) The volume occupied by 0.25 mole of O₂ gas at STP

(c) The number of atoms in 48 g of magnesium

(Atomic masses: C = 12 u, Mg = 24 u; Avogadro's constant = 6.022 × 10²³ mol⁻¹)

Q15. During a chemistry practical class, you are given four containers:

Container A: 16 g of oxygen gas (O₂)

Container B: 16 g of methane gas (CH₄)

Container C: 16 g of sulphur dioxide gas (SO₂)

Container D: 16 g of hydrogen gas (H₂)

All containers are at the same temperature and pressure.

(a) Calculate the number of moles of gas in each container.

(b) Which container has the maximum number of molecules? Explain.

(c) Which container occupies the largest volume at STP? Calculate the volume.

(d) Your teacher asks you to mix Container A and Container D. What would be the total number of moles in the mixture? Would the volume be simply additive? Explain.

Q16. Calculate:

(a) The number of moles in 11.2 L of CO₂ gas at STP

(b) The number of molecules in 0.5 mole of NH₃

(c) The mass of 1.5055 × 10²³ molecules of H₂SO₄

(Atomic masses: H = 1 u, C = 12 u, N = 14 u, O = 16 u, S = 32 u; Avogadro's constant = 6.022 × 10²³ mol⁻¹)

Q17. A sample of glucose (C₆H₁₂O₆) weighs 360 g.

(a) Calculate the number of moles of glucose.

(b) Calculate the number of molecules of glucose.

(c) Calculate the number of carbon atoms, hydrogen atoms, and oxygen atoms in the sample.

(d) If this glucose is dissolved in water to make 2 L of solution, what is the molarity of the solution?

(Atomic masses: C = 12 u, H = 1 u, O = 16 u)

Q18. A chemistry teacher asks you to prepare exactly 1 mole of sodium chloride (NaCl) for a demonstration.

(a) Calculate the mass of NaCl required.

(b) If you dissolve this mass in 500 mL of water, what is the molarity of the solution?

(c) How many Na⁺ ions and Cl⁻ ions are present in the solution?

(d) If you had to prepare 0.1 mole of NaCl instead, how many formula units would you need?

(Atomic masses: Na = 23 u, Cl = 35.5 u)