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1.4.5Additional lessons

Avogadro’s Law — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 18 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين المول والحسابات الكيميائية - Moles & Stoichiometry FormulasFast Exam Solution
n = m / M_r | N = n × N_A (6.022 × 10²³) | M = n / V | V_gas = n × 22.4 L (STP)
Word Equation & Conditions: عدد المولات = الكتلة بالجرام ÷ الكتلة المولية | عدد الجسيمات = عدد المولات × عدد أفوجادرو
Location & Specifications: كيمياء التحصيلي وثانوي وبكالوريا و Cambridge IGCSE Chemistry.
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Section 1 of 5The Law

Equal volumes of different gases contain equal numbers of molecules under the same conditions.

Statement
  • Statement At the same temperature and pressure, equal volumes of all gases contain the same number of particles.
Proportion
  • Proportion Volume is directly proportional to amount of gas, so V ∝ n, or V ÷ n is constant.
Condition
  • Condition Temperature and pressure must be constant and the same for all the gases compared.
Reason
  • Reason Gas particles are far apart, so the volume depends on how many there are rather than on their size.

Section 2 of 5Applying to Reactions

For gases at the same temperature and pressure, volume ratios equal mole ratios.

1
Write the balanced equation, including state symbols, to see the mole ratio of the gases.
2
Replace mole ratio by volume ratio, since each mole occupies the same volume.
3
Use the ratio to find the volume of a gaseous product or a reagent needed.
4
For 2H₂(g) + O₂(g) → 2H₂O(g), 20 cm³ of hydrogen needs 10 cm³ of oxygen and gives 20 cm³ of steam.
5
Ignore liquids and solids, because the law applies only to gases.

Section 3 of 5Limits and Uses

Limits and Uses
Real gases
  • Real gases The law is exact for an ideal gas and is a good approximation for real gases at low pressure and high temperature.
Molar volume
  • Molar volume The same volume per mole means one value can be used for any gas under given conditions.
Comparison
  • Comparison Samples of different gases of equal volume may differ greatly in mass.
Reaction yields
  • Reaction yields The law lets gas volumes in reactions be compared without knowing the actual number of moles.

Section 4 of 5Worked Example

Question

  • 30 cm³ of methane is burned in excess oxygen: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l).
  • What volume of oxygen reacts and what volume of CO₂ forms, measured under the same conditions?

  1. Mole ratio CH₄O₂ : CO₂ = 1 : 2 : 1.
  2. Volume of O₂ = 2 × 30 = 60 cm³.
  3. Volume of CO₂ = 30 cm³; water is a liquid so it does not count.

Answer 60 cm³ of O₂ reacts and 30 cm³ of CO₂ forms

Section 5 of 5Exam Checklist & Tips

1.4.5 – Key Points for the Exam

  • Avogadro's law At constant temperature and pressure the volume of a gas is proportional to its amount.
  • Ideal gas A model gas whose particles have no volume and no forces between them.
  • Gas volume ratio The ratio of gas volumes in a reaction at the same temperature and pressure.
  • Stoichiometry The quantitative relationship between reactants and products in an equation.

Common Mistakes

  • Applying the volume ratio to a liquid product such as water at room temperature.
  • Assuming equal volumes of gases have equal masses.
  • Using the law when temperature or pressure differs between the gases.
  • Using mass ratios from the equation as if they were volume ratios.

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