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AQA GCSE · Question 05.4 · Quantitative chemistry
Air is 20% oxygen.
Calculate the volume of air needed to provide enough oxygen to react with 3.50 dm³ of hydrogen gas.
The equation for the reaction is:
2H₂ + O₂ → 2H₂O
Air is 20% oxygen.
Calculate the volume of air needed to provide enough oxygen to react with 3.50 dm³ of hydrogen gas.
The equation for the reaction is:
2H₂ + O₂ → 2H₂O
How to approach this question
This is a two-step reacting gas volume calculation.
1. **Find the volume of oxygen:** Look at the balanced chemical equation. The numbers in front (stoichiometric coefficients) give the ratio of volumes for gases. Determine the ratio between hydrogen (H₂) and oxygen (O₂). Use this ratio to calculate the volume of pure oxygen needed to react with 3.50 dm³ of hydrogen.
2. **Find the volume of air:** You are told that air is 20% oxygen. The volume of oxygen you just calculated is 20% of the total volume of air. To find the total volume of air, you need to scale up the oxygen volume. You can do this by dividing the volume of oxygen by 0.20 (which is the same as multiplying by 5).
Full Answer
**Step 1: Use the molar ratio from the balanced equation to find the volume of oxygen needed.**
The ratio of H₂ to O₂ is 2 : 1.
This means the volume of oxygen needed is half the volume of hydrogen.
Volume of O₂ = Volume of H₂ / 2
Volume of O₂ = 3.50 dm³ / 2
Volume of O₂ = 1.75 dm³
**Step 2: Calculate the volume of air required.**
Air is 20% oxygen. This means Volume of O₂ = 0.20 × Volume of air.
To find the volume of air, rearrange the formula:
Volume of air = Volume of O₂ / 0.20
Volume of air = 1.75 dm³ / 0.20
Volume of air = 8.75 dm³
This problem involves using molar ratios of gases and percentage composition.
**Part 1: Molar Ratio**
The balanced equation is 2H₂ + O₂ → 2H₂O.
According to Avogadro's law, for gases at the same temperature and pressure, the ratio of volumes is the same as the ratio of moles.
The molar ratio of H₂ : O₂ is 2 : 1.
This means that for every 2 volumes of hydrogen, 1 volume of oxygen is required.
So, the volume of oxygen needed is half the volume of hydrogen.
Volume of O₂ = 3.50 dm³ / 2 = 1.75 dm³.
**Part 2: Percentage Composition**
We need 1.75 dm³ of pure oxygen. We are told that air is 20% oxygen by volume.
Let V_air be the total volume of air.
Then, the volume of oxygen is 20% of V_air.
Volume of O₂ = 0.20 × V_air
We can rearrange this to find the volume of air:
V_air = Volume of O₂ / 0.20
V_air = 1.75 / 0.20
V_air = 8.75 dm³.
Alternatively, since 20% is 1/5, the volume of air is 5 times the volume of oxygen:
V_air = 1.75 × 5 = 8.75 dm³.
Common mistakes
✗ Getting the molar ratio wrong (e.g., 1:1 or 1:2).
✗ Forgetting to calculate the volume of air from the volume of oxygen.
✗ Incorrectly calculating the volume of air (e.g., multiplying the oxygen volume by 0.20 instead of dividing).
Practice the full AQA GCSE Chemistry Higher Tier Paper 2
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