Question 1 Medium
In 2H₂ + O₂ → 2H₂O, what volume of oxygen reacts with 60 cm³ of hydrogen?
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Answer: 30 cm³
- The ratio of H₂ to O₂ is 2 : 1.
- Half of 60 is 30.
Gas volumes: GCSE Combined Science Chemistry practice questions
A second set of questions on gas volumes. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
N₂ + 3H₂ → 2NH₃. What volume of ammonia is made from 30 dm³ of nitrogen with excess hydrogen?
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Answer: 60 dm³
- The ratio N₂ to NH₃ is 1 : 2.
- 30 × 2 = 60 dm³.
Question 3 Medium
Calculate the number of moles in 2400 cm³ of a gas at room temperature and pressure.
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Answer: 0.10 mol
- Use 24 000 cm³ per mole.
- 2400 ÷ 24 000 = 0.10 mol.
Question 4 Medium
In CH₄ + 2O₂ → CO₂ + 2H₂O, what volume of oxygen reacts with 50 cm³ of methane? All gases are at the same temperature and pressure.
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Answer: 100 cm³
- The ratio of CH₄ to O₂ is 1 : 2.
- Double 50 cm³ is 100 cm³.
Question 5 Hard
Calculate the mass of calcium carbonate needed to make 1.2 dm³ of carbon dioxide at room temperature and pressure: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Mr of CaCO₃ = 100.
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Answer: 5.0 g
- Moles of CO₂ = 1.2 ÷ 24 = 0.050.
- Moles of CaCO₃ = 0.050.
- Mass = 0.050 × 100 = 5.0 g.
Question 6 Hard
2H₂O₂ → 2H₂O + O₂. Calculate the volume of oxygen made when 3.4 g of hydrogen peroxide decomposes. Mr of H₂O₂ = 34.
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Answer: 1.2 dm³
- Moles of H₂O₂ = 3.4 ÷ 34 = 0.10.
- Moles of O₂ = 0.050.
- Volume = 0.050 × 24 = 1.2 dm³.
Question 7 Hard
100 cm³ of hydrogen and 100 cm³ of oxygen react to make steam: 2H₂ + O₂ → 2H₂O. What volume of oxygen is left over?
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Answer: 50 cm³
- 100 cm³ of hydrogen reacts with 50 cm³ of oxygen.
- 100 − 50 = 50 cm³ left.
Question 8 Hard
Calculate the mass of 6.0 dm³ of carbon dioxide at room temperature and pressure. Mr of CO₂ = 44.
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Answer: 11 g
- Moles = 6.0 ÷ 24 = 0.25.
- Mass = 0.25 × 44 = 11 g.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What does Avogadro’s law say?
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Equal volumes of gases at the same temperature and pressure have the same number of molecules.
Check 2
What is the molar volume at room temperature and pressure?
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24 dm³ per mole.
Check 3
What is 24 dm³ in cm³?
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24 000 cm³.
Check 4
What is room temperature and pressure?
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20 °C and 1 atmosphere.
Check 5
How do you find the volume of a gas from moles?
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Multiply moles by 24 for dm³.
Check 6
How do you find the moles of a gas from its volume in dm³?
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Divide the volume by 24.
Check 7
How do you find the moles of a gas from its volume in cm³?
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Divide the volume by 24 000.
Check 8
What is the volume of 0.5 mol of any gas at rtp?
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12 dm³.
Check 9
What do gas volume ratios match in an equation?
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The ratio of the big numbers.
Check 10
What volume of O₂ reacts with 100 cm³ of H₂ in 2H₂ + O₂ → 2H₂O?
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50 cm³.
Check 11
Why does the size of a gas molecule not matter for volume?
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The molecules are very far apart.
Check 12
What is the first step to find a gas volume from a mass?
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Convert the mass to moles.
Check 13
How many cm³ is 2.4 dm³?
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2400 cm³.
Check 14
What is the volume of 1 mol of hydrogen at rtp?
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24 dm³.
Check 15
What is the volume of 1 mol of carbon dioxide at rtp?
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24 dm³, the same as any gas.
Check 16
What does rtp stand for?
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Room temperature and pressure.
Check 17
How many cm³ are there in 1 dm³?
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1000 cm³.
Check 18
What is the volume of 0.25 mol of any gas at rtp?
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6 dm³.
Check 19
What is the volume of 2 mol of any gas at rtp?
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48 dm³.
Check 20
How many moles are in 48 dm³ of any gas at rtp?
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2 mol.
Check 21
What is the second step to find a gas volume from a mass?
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Use the equation ratio to find the moles of gas.
Check 22
What is the last step to find a gas volume from a mass?
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Multiply the moles of gas by 24.
Check 23
What does it mean if a gas is in excess?
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There is more than enough, so some is left over at the end.
Check 24
What must be true for gas volume ratios to work?
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The gases must be measured at the same temperature and pressure.
Keep going with Gas volumes
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