Moles and Volumes of Gases
Recap Moles and Volumes of Gases for GCSE Chemistry with this free worksheet and full mark scheme — Higher-tier exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) Higher topic: at room temperature and pressure one mole of any gas occupies 24 dm³.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
Equal amounts in moles of any gases occupy the same volume under the same conditions of temperature and pressure. This is a Triple and Higher-only topic.
At room temperature and pressure, one mole of any gas occupies 24 dm³. This is called the molar gas volume, and it applies regardless of which gas is involved, which is a surprising but useful result.
The formula is volume of gas in dm³ = moles × 24. It rearranges to moles = volume ÷ 24. As with concentration calculations, volumes given in cm³ must be converted to dm³ first by dividing by 1000.
Revision notes
The molar gas volume
One mole of any gas occupies 24 dm³ at room temperature and pressure.
This holds regardless of the identity of the gas, because equal amounts in moles of any gases occupy the same volume under the same conditions.
The calculation
Volume in dm³ = moles × 24.
Rearranged: moles = volume ÷ 24. If the volume is given in cm³, divide by 1000 first to convert it to dm³.
Combining with reacting masses
Many questions combine this with a reacting mass calculation.
Convert the mass of a reactant into moles, use the equation ratio to find the moles of gas produced, then multiply by 24 to find its volume.
Key points
- Equal moles of any gas occupy equal volumes.
- One mole of gas occupies 24 dm³ at room temperature and pressure.
- Volume in dm³ = moles × 24.
- Moles = volume ÷ 24.
- Convert cm³ to dm³ by dividing by 1000.
- The gas identity does not affect the volume.
Worked examples
Example 1
Calculate the volume occupied by 0.5 moles of carbon dioxide at room temperature and pressure. [2 marks]
Model answer
Volume = moles × 24 = 0.5 × 24 (1 mark) = 12 dm³ (1 mark).
Example 2
Calculate the number of moles in 72 dm³ of oxygen at room temperature and pressure. [2 marks]
Model answer
Moles = volume ÷ 24 = 72 ÷ 24 (1 mark) = 3 moles (1 mark).
Example 3
Explain why 1 mole of hydrogen and 1 mole of carbon dioxide occupy the same volume. [2 marks]
Model answer
Equal amounts in moles of any gas contain the same number of particles (1 mark), and under the same conditions of temperature and pressure those particles occupy the same volume regardless of the gas (1 mark).
Common mistakes
Using 24 for a volume in cm³.
The molar gas volume is 24 dm³, so convert cm³ to dm³ first.
Thinking heavier gases occupy more volume.
Equal moles of any gas occupy the same volume under the same conditions.
Multiplying when you should divide.
Moles = volume ÷ 24; volume = moles × 24.
Forgetting the conditions.
The value 24 dm³ applies at room temperature and pressure.
Exam tips
- State room temperature and pressure when quoting 24 dm³.
- Convert volumes to dm³ before substituting.
- Check whether you need to multiply or divide.
- Remember this is a Triple and Higher-only topic.
Key terms
- Molar gas volume
- The volume of one mole of gas, 24 dm³ at rtp.
- Room temperature and pressure
- The standard conditions for the 24 dm³ value.
- Cubic decimetre
- A volume of 1000 cm³.
- Mole
- The unit of amount of substance.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.