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5.1.3Additional lessons

Avogadro’s Law and Molar Volume of Gas — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 91 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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Part 1 of 5Avogadro's Law

Statement
  • Statement Equal volumes of all gases at the same temperature and pressure contain the same number of molecules.
Proportionality
  • Proportionality At constant temperature and pressure, the volume of a gas is directly proportional to the number of moles.
Gas volume ratios
  • Gas volume ratios The ratio of volumes of gases in a reaction equals the ratio of their coefficients in the balanced equation.
Ideal behaviour
  • Ideal behaviour The law works best for gases at high temperature and low pressure, where they behave most like ideal gases.

Part 2 of 5Molar Volume and STP

Always check the stated conditions before choosing the molar volume.

1
The molar volume is the volume occupied by one mole of an ideal gas under stated conditions.
2
In IB, standard temperature and pressure are 273 K and 100 kPa, where molar volume is 22.7 dm³ mol⁻¹.
3
To convert, use n = V ÷ 22.7 when volume is in dm³, or V = n × 22.7 to find volume from moles.
4
If the volume is in cm³, divide by 1000 to convert to dm³ before using the molar volume.
5
At other temperatures and pressures, use the ideal gas equation PV = nRT instead.

Part 3 of 5Applications

Applications
Reaction volumes
  • Reaction volumes Gas volumes can be compared directly, so 2 dm³ H₂ reacts with 1 dm³ O₂ to give 2 dm³ steam at the same conditions.
Gas evolution
  • Gas evolution The volume of CO₂ from carbonates and acids is used to find amounts of carbonate.
Density
  • Density Density of a gas = M ÷ molar volume, linking mass and volume.
Limitations
  • Limitations Real gases deviate from ideal behaviour near condensation or at very high pressure.

Part 4 of 5Worked Example

Question

  • Calculate the volume of carbon dioxide at STP produced when 5.00 g of calcium carbonate reacts with excess hydrochloric acid: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g).
  • (M of CaCO₃ = 100.09 g mol⁻¹;
    • molar volume = 22.7 dm³ mol⁻¹)

  1. n(CaCO₃) = 5.00 ÷ 100.09 = 0.04996 mol.
  2. Mole ratio CaCO₃CO₂ = 1 : 1, so n(CO₂) = 0.04996 mol.
  3. V = 0.04996 × 22.7 = 1.13 dm³.

Answer 1.13 dm³ of CO₂ (1130 cm³)

Part 5 of 5Exam Checklist & Tips

5.1.3 – Key Points for the Exam

  • Avogadro's law Equal volumes of gases under the same conditions contain equal numbers of molecules.
  • Molar volume The volume of one mole of an ideal gas, 22.7 dm³ at 273 K and 100 kPa.
  • STP Standard temperature and pressure, 273 K and 100 kPa in IB.
  • Ideal gas A gas whose particles have negligible volume and no attractions.

Common Mistakes

  • Using 22.4 dm³ mol⁻¹ when the question's stated pressure and temperature require a different value.
  • Using the molar volume for liquids or solids.
  • Forgetting to convert cm³ to dm³ before multiplying by 22.7.
  • Applying the molar volume at room temperature without checking the conditions given.

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