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Difficulty: EasyComposition and Pollution of Air

A 100 cm3100\text{ cm}^3 sample of dry air is passed slowly over excess heated copper turnings in a combustion tube until no further contraction in volume occurs. Assuming oxygen accounts for 21%21\% of air by volume, what is the volume of the remaining unreacted gas in cm3\text{cm}^3 at room temperature and pressure?

Answer: 79 cm^3

Answer

The volume of the remaining unreacted gas is 79 cm379\text{ cm}^3.
Dry atmospheric air is made up of approximately 21%21\% oxygen gas and 79%79\% non-reactive gases (primarily nitrogen along with argon and carbon dioxide). When passed over heated copper, only oxygen reacts to form solid copper(II) oxide. The volume of oxygen removed is 21 cm321\text{ cm}^3, leaving 100 cm3−21 cm3=79 cm3100\text{ cm}^3 - 21\text{ cm}^3 = 79\text{ cm}^3 of unreacted gaseous mixture.

Step-by-Step Solution

1
Calculate the volume of oxygen absorbed by the heated copper
Volume of O2=21100×100 cm3=21 cm3\text{O}_2 = \frac{21}{100} \times 100\text{ cm}^3 = 21\text{ cm}^3
Air contains approximately 21%21\% oxygen by volume, which reacts quantitatively with heated copper turnings.
2
Determine the remaining unreacted gas volume
Volume remaining = 100 cm3−21 cm3=79 cm3100\text{ cm}^3 - 21\text{ cm}^3 = 79\text{ cm}^3
The unreacted component consists mainly of nitrogen (approx. 78%78\%) and noble gases (approx. 1%1\%) which do not react with heated copper.

Key Concept

Composition of dry air and quantitative removal of oxygen gas
Estimated Time:45s
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Topics

Composition and Pollution of Air

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