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1.5.10States of Matter

Calculate Volumes of Gases — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 17 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Avogadro's Law and Molar Volume

Gas volumes can be converted to moles because equal amounts of gas occupy equal volumes under the same conditions.

Avogadro's Law
  • Avogadro's Law Equal volumes of gases at the same temperature and pressure contain the same number of molecules, and so the same number of moles.
Molar gas volume
  • Molar gas volume The volume occupied by one mole of any gas at room temperature and pressure, which is 24 dm³ or 24 000 cm³.
RTP
  • RTP Room temperature and pressure, defined in this topic as 20 °C and 1 atmosphere.
Units
  • Units Use 24 000 cm³ per mole instead of 24 dm³ per mole when the volume of gas is given in cm³.

Part 2 of 5Calculating Gas Volumes

1
Find the moles of gas from its mass by dividing the mass by the Mr of the gas.
2
Multiply the moles by 24 to give the volume in dm³ at room temperature and pressure.
3
Multiply the moles by 24 000 instead to give the volume in cm³ at room temperature and pressure.
4
Divide a known gas volume by 24 to find the moles in dm³, which reverses the calculation when the amount is unknown.

Part 3 of 5Gas Reactions

Gas Reactions
Equation ratio
  • Equation ratio The coefficients in a balanced equation give the ratio of gas volumes,
    • so 2 volumes of hydrogen react with 1 volume of oxygen to form water.
Liquids excluded
  • Liquids excluded Water is a liquid at room temperature and pressure, so it does not contribute to the gas volumes of the products.
Units
  • Units Keep the volumes in the units the question asks for, converting between cm³ and dm³ at the end if necessary.
Methane burning
  • Methane burning CH₄ + 2O₂ → CO₂ + 2H₂O means 50 cm³ of methane needs 100 cm³ of oxygen and forms 50 cm³ of carbon dioxide.

Part 4 of 5Worked Example

Question

  • What volume does 154 g of nitrogen gas occupy at room temperature and pressure?
  • Use Ar N = 14.

  1. Mr of N₂ = 2 × 14 = 28.
  2. Moles = 154 ÷ 28 = 5.5 mol.
  3. Volume = 5.5 × 24 = 132 dm³.

Answer 132 dm³ (which is 132 000 cm³).

Part 5 of 5Exam Checklist & Tips

1.5.10 – Key Points for the Exam

  • Avogadro's Law Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
  • Molar gas volume The volume occupied by one mole of a gas at room temperature and pressure.
  • RTP Room temperature and pressure, 20 °C and 1 atmosphere.
  • Combustion Burning a substance in oxygen.

Common Mistakes

  • Using 22.4 dm³ per mole, which is the value at standard temperature and pressure, not at room temperature and pressure.
  • Forgetting to use 24 000 when the volume is in cm³, or to convert units at the end of the calculation.
  • Including water in the gas volumes of products; water is a liquid under these conditions.
  • Using the mass in grams directly as the number of moles without dividing by the Mr.

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