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Combustion gas volume calculation
Air is 20% oxygen. The equation for the reaction is:
Calculate the volume of air needed to provide enough oxygen to react with 3.50 dm³ of hydrogen gas.
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Marking points
- 1 Uses the balanced equation to find an oxygen volume of 1.75 dm³ from 3.50 dm³ hydrogen.
- 2 Uses the 20% oxygen content to scale the oxygen volume by 100/20.
- 3 Gives 8.75 dm³ for the volume of air.
Full-mark answer
From 2 H₂ : 1 O₂, oxygen volume = 3.50 ÷ 2 = 1.75 dm³. Air volume = 1.75 × 100/20 = 8.75 dm³.
Why this answer loses marks
The student treats the hydrogen volume as the required volume of air.
The equation ratio must first give the oxygen requirement, and the oxygen percentage must then be used to find total air.