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6.4.5Chemical Reactions

Gas Volumes — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 9 of 21 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Section 1 of 5The Molar Gas Volume

6.4.5 Gas Volumes

Equal amounts of any gas occupy the same volume under the same conditions.

Molar gas volume
  • The volume occupied by one mole of any gas at room temperature and pressure, r.t.p., is 24 dm³.
In cm³
  • The same volume in cm³ is 24 000 cm³ per mole, so the volume is larger in cm³.
Room conditions
  • Room temperature and pressure is taken as about 20 °C and 1 atmosphere in this calculation.
Any gas
  • The molar gas volume applies to all gases, because equal numbers of moles of any gases occupy the same volume under the same conditions.

Section 2 of 5Converting Moles and Volume

Converting between moles and volume only needs one multiplication or division.

1
Identify whether the question gives moles or volume, and whether the volume is in dm³ or cm³.
2
To find the volume from moles, multiply the number of moles by 24 for dm³, or by 24 000 for cm³.
3
To find moles from a volume in dm³, divide the volume by 24; for a volume in cm³, divide by 24 000.
4
Check the answer is sensible: a larger number of moles should always give a larger volume.

Section 3 of 5Common Pitfalls

Common Pitfalls
Unit mismatch
  • A volume in cm³ must be divided by 24 000, not 24, to find the number of moles.
Molar volume
  • The molar gas volume at r.t.p. is 24 dm³, which differs from the value 22.4 dm³ used at s.t.p.
Mass versus volume
  • The volume of a gas depends on the number of moles, not on the mass of gas, so mass must be converted first.
Units
  • Gas volumes are usually given in dm³ or cm³, so the unit should always be written in the answer.

Section 4 of 5Worked Example

Question

  • (a) Calculate the volume, in dm³, occupied by 0.25 mol of carbon dioxide at r.t.p.
  • (b) Calculate the number of moles in 360 cm³ of hydrogen at r.t.p.

  1. (a) Volume = 0.25 × 24 = 6.0 dm³.
  2. (b) Convert the volume to moles: moles = 360 ÷ 24 000 = 0.015 mol.
  3. The answers are 6.0 dm³ for carbon dioxide and 0.015 mol for hydrogen.

Answer (a) 6.0 dm³; (b) 0.015 mol

Section 5 of 5Exam Checklist & Tips

6.4.5 – Key Points for the Exam

  • Molar gas volume The volume occupied by one mole of any gas at r.t.p., equal to 24 dm³.
  • r.t.p. Room temperature and pressure, the standard conditions used for gas volume calculations.
  • Gas volume The space occupied by a gas, measured in dm³ or cm³.
  • Amount of substance The number of moles of a substance, which is directly related to the volume of a gas.

Common Mistakes

  • Using 24 when the volume is in cm³, which should be divided by 24 000.
  • Using the value 22.4 dm³, which applies to s.t.p., rather than 24 dm³ at r.t.p.
  • Dividing by the molar gas volume when the question asks for volume, which should be multiplied.
  • Forgetting to convert a mass of gas into moles before calculating its volume.

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