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CHAPTER 3 • 4 MARKS

Chemical Calculations

Learn atomic mass, molecular mass, mole concept, Avogadro number and stoichiometric calculations.

Topics in this chapter

  1. Relative atomic mass
  2. Molecular and formula mass
  3. Mole and molar mass
  4. Avogadro number
  5. Molar volume of gases
  6. Percentage composition
  7. Stoichiometric calculations from equations

Key learning points

✓ One mole contains 6.022 × 10²³ entities.

✓ Molar mass in grams is numerically equal to relative molecular or formula mass.

✓ A balanced equation gives the reacting mole ratio.

✓ Limiting-reactant ideas begin with comparing available mole ratios.

Important equations and relations

n = m/MN = nNₐAt STP, 1 mol gas ≈ 22.4 L2H₂ + O₂ → 2H₂O

Multiple-choice questions

1. One mole contains:
  1. 6.022 × 10²³ particles
  2. 22.4 particles
  3. 1 particle
  4. 273 particles

Answer: A — 6.022 × 10²³ particles

2. The molar mass of O₂ is:
  1. 16 g mol⁻¹
  2. 18 g mol⁻¹
  3. 32 g mol⁻¹
  4. 44 g mol⁻¹

Answer: C — 32 g mol⁻¹

3. In 2H₂ + O₂ → 2H₂O, the mole ratio H₂:O₂ is:
  1. 1:1
  2. 2:1
  3. 1:2
  4. 2:2

Answer: B — 2:1

Very short-answer questions

  1. Define one mole.
  2. Write Avogadro’s number.
  3. Calculate the molecular mass of H₂O.

Short-answer questions

  1. Calculate the number of moles in 11 g of CO₂.
  2. Find the mass of 0.5 mol of NaOH.

Long-answer question

  1. Explain how a balanced chemical equation is used to solve mass-to-mass stoichiometric problems.