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1.4.5Additional lessons

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

5 min read 5 parts Lesson 15 of 98

📚 IB Chemistry SL 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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Part 1 of 5The Law

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

Statement
  • Statement At the same temperature and pressure, equal volumes of all gases contain equal numbers of particles.
Proportionality
  • Proportionality At constant temperature and pressure the volume of a gas is directly proportional to its amount in moles.
Mole ratio
  • Mole ratio Therefore the ratio of volumes of gases in a reaction is the same as the ratio of their coefficients.
Conditions
  • Conditions The law holds only when all volumes are measured at the same temperature and pressure.

Part 2 of 5Using It in Reactions

Gas volume ratios can replace mole ratios when everything is a gas.

1
Write the balanced equation with state symbols so that you can see which species are gases.
2
Read the coefficients of the gases to get the volume ratio, ignoring solids and liquids.
3
Multiply the given volume by the ratio to obtain the volume of the other gas.
4
State that the volumes were measured under the same conditions of temperature and pressure.

Part 3 of 5Limits

Limits
Liquid water
  • Liquid water Water produced below its boiling point is a liquid and takes up almost no volume compared with gases.
Real gases
  • Real gases At high pressure or low temperature, gases deviate and the proportionality is only approximate.
Different volumes
  • Different volumes Volumes at different pressures or temperatures cannot be compared by the simple ratio.
Identity of gas
  • Identity of gas The law does not depend on the type of gas, so a mole of any ideal gas occupies the same volume.

Part 4 of 5Worked Example

Question

  • 20.0 cm³ of methane burns completely in excess oxygen: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l).
  • All volumes are measured at the same conditions.
  • Find the volume of oxygen used and of carbon dioxide formed.

  1. The gas ratio CH₄O₂ : CO₂ = 1 : 2 : 1.
  2. Oxygen used = 2 × 20.0 = 40.0 cm³.
  3. Carbon dioxide formed = 1 × 20.0 = 20.0 cm³; water is liquid and does not count.

Answer

  • 40.0 cm³ of O₂ used;
    • 20.0 cm³ of CO₂ formed

Part 5 of 5Exam Checklist & Tips

1.4.5 – Key Points for the Exam

  • Avogadro's law Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
  • Gas volume ratio The ratio of volumes of reacting gases, equal to the ratio of coefficients in the equation.
  • Standard conditions Specified temperature and pressure at which gas volumes are compared.
  • Direct proportion A relationship in which one quantity doubles when the other doubles.

Common Mistakes

  • Applying the volume ratio to solids or liquids in the equation.
  • Comparing gas volumes measured at different temperatures or pressures.
  • Thinking that equal volumes of gases have equal masses.
  • Counting liquid water as a gas when the products are cooled.

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