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1.5.2Additional lessons

Molar Gas Volume — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 19 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 5Molar Volume

By Avogadro's law one mole of any gas occupies the same volume at a given temperature and pressure.

Definition
  • Definition The molar gas volume is the volume occupied by one mole of gas under stated conditions.
STP value
  • STP value At standard temperature and pressure, 273 K and 100 kPa, the molar volume is 22.7 dm³ mol⁻¹.
Equation
  • Equation Amount of gas n = V ÷ 22.7 when V is in dm³ at STP.
Units
  • Units If the volume is in cm³, divide by 1000 first to convert to dm³.

Section 2 of 5Using Molar Volume

These steps solve most reaction problems involving gas volumes at STP.

1
Write a balanced equation and identify the substance with known mass or volume.
2
Convert the known quantity to moles, using M for masses or 22.7 dm³ mol⁻¹ for gas volumes.
3
Use the mole ratio in the equation to find the moles of the gas wanted.
4
Convert moles to volume by multiplying by 22.7 dm³ mol⁻¹.
5
Check that the units are consistent and that the conditions are STP.

Section 3 of 5Conditions and Limits

Conditions and Limits
Other conditions
  • Other conditions At room temperature and pressure the molar volume is larger, so the STP value cannot be used unless stated.
Ideal gas equation
  • Ideal gas equation For other conditions, use PV = nRT, which works for any temperature and pressure.
Assumption
  • Assumption The calculation treats gases as ideal, so it is most accurate at low pressure and high temperature.
Data
  • Data The value is given in the IB data booklet and should be quoted with its conditions.
GasAmount / molVolume at STP / dm³
Any gas1.0022.7
Any gas0.50011.35
Any gas2.0045.4

Section 4 of 5Worked Example

Question

  • 0.486 g of magnesium reacts with excess hydrochloric acid: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
  • Calculate the volume of hydrogen at STP.

  1. n(Mg) = 0.486 ÷ 24.31 = 0.0200 mol.
  2. The mole ratio MgH₂ is 1 : 1, so n(H₂) = 0.0200 mol.
  3. V = 0.0200 × 22.7 = 0.454 dm³.

Answer 0.454 dm³ (454 cm³) of hydrogen

Section 5 of 5Exam Checklist & Tips

1.5.2 – Key Points for the Exam

  • Molar gas volume The volume occupied by one mole of gas at stated conditions.
  • STP Standard temperature and pressure: 273 K and 100 kPa.
  • Mole ratio The ratio of amounts of substances in a balanced equation.
  • Ideal gas A model gas obeying PV = nRT exactly.

Common Mistakes

  • Using 22.7 dm³ mol⁻¹ when the gas is at room temperature rather than STP.
  • Leaving the volume in cm³ and forgetting to convert to dm³.
  • Using the 22.7 value for a liquid or solid product.
  • Forgetting to use the mole ratio from the equation before calculating the gas volume.

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