Chapter 5 · ICSE Class 10 Chemistry (Selina Concise)

Mole Concept and Stoichiometry

Chemists count atoms by weighing them. The mole is the bridge between the tiny world of particles and the grams on a balance.

⚖️ Chapter 5

The mole at the centre of mass, volume and number of particles Moles convert to mass by multiplying by molar mass, to gas volume at STP by multiplying by 22.4 litres, and to number of particles by multiplying by Avogadro's number 6.022 times 10 to the 23. 1 mole Mass molar mass in g Gas volume 22.4 L at STP Particles 6.022 × 10²³
5.1

The Mole, Avogadro's Law and Molar Volume

Avogadro's law states that equal volumes of all gases, under the same temperature and pressure, contain the same number of molecules. One mole of any substance contains 6.022 × 10²³ particles. It has a mass equal to its molar mass in grams, and one mole of any gas occupies 22.4 litres at STP. Vapour density is half the molecular mass.

1

What is the mass of 2 moles of water? (H = 1, O = 16)

Molar mass of H₂O = 18 g, so 2 × 18 = 36 g

2

What volume do 3 moles of a gas occupy at STP?

3 × 22.4 = 67.2 litres

3

How many molecules are present in 0.5 mole of a substance?

0.5 × 6.022 × 10²³ = 3.011 × 10²³ molecules

4

The vapour density of CO₂ is 22. What is its molecular mass?

Molecular mass = 2 × vapour density = 44

5.2

Calculations from Chemical Equations

A balanced equation shows the ratio of moles, and for gases the ratio of volumes, in which substances react. Use it to find masses and volumes of products. (Atomic masses: H = 1, C = 12, N = 14, O = 16, Ca = 40.)

1

In N₂ + 3H₂ → 2NH₃, what volume of ammonia forms from 10 litres of nitrogen with excess hydrogen?

The volume ratio N₂ : NH₃ is 1 : 2, so 20 litres of NH₃ form

2

What mass of CO₂ forms when 100 g of calcium carbonate is heated? CaCO₃ → CaO + CO₂

100 g is 1 mole of CaCO₃, which gives 1 mole of CO₂, so the mass is 44 g (22.4 litres at STP)

3

A compound is 40% C, 6.7% H and 53.3% O. Find its empirical formula.

The mole ratio is C : H : O = 3.33 : 6.7 : 3.33, which is 1 : 2 : 1. The empirical formula is CH₂O

Practice

Exercises

1. Calculate the molecular mass of CO₂.

2. What is the mass of 3 moles of ammonia, NH₃?

3. What volume does 0.5 mole of any gas occupy at STP?

4. How many molecules are in 2 moles of a substance?

5. State Avogadro's law.

6. A gas has a vapour density of 22. What is its molecular mass?

7. How many moles are there in 22 g of CO₂?

8. In N₂ + 3H₂ → 2NH₃, what volume of ammonia is made from 10 litres of nitrogen with excess hydrogen?

9. What mass of CO₂ is obtained by heating 50 g of CaCO₃?

10. What is the percentage of oxygen by mass in water?

Next chapter — Electrolysis →