Question

Air contains 23% oxygen and 77% nitrogen by weight. The percentage of O2 by volume is(1) 28.1(2) 20.7(3) 21.8(4) 23.0

Answer 1

The correct answer is option 2 - 20.7 %.

Assume that the calculations are for 100 g of air:-

Mass of oxygen (O₂) in 100 g of air = 23 g

Mass of nitrogen (N₂) in 100 g of air = 77 g

Molar mass of Oxygen = 32 g

1 mole of oxygen or 32 g of Oxygen occupies 22.4 L volume.

Therefore, volume of 23 g of oxygen will occupy 22.4/32 x 23 = 16.1 L

Similarly, 28 g of Nitrogen occupies 22.4 L volume.

Therefore, 77 g of Nitrogen will occupy 22.4/28 x 77 = 61.6 L

Total volume of air = 16.1 + 61.6 = 77.7 L

% by volume of Oxygen = 16.1/77.7 x 100 = 20.7 %

Answer 2

As the mass of oxygen in air is 23% and of nitrogen is 77%
So we can say that in 100 gram of air oxygen will be 23 grams and nitrogen will be 77 grams by mass.

We also know that molar mass oxygen is = 32 g
So at Standard temperature and pressure 32 grams or 1 mole of oxygen = 22.4 L
Hence the 23 gram of oxygen = 22.4/32 *23 = 16.1 liter

Also we know that molar mass of nitrogen = 28 grams
So the 28 gram nitrogen will contain the volume = 22.4 L
hence,, the 77 grams of nitrogen contain volume =22.4/2 *77 = 61.6 liter

So the total volume the air will have= 61.6+16.1 = 77.7 Liters
And the percentage of air in that volume = 16.1/77.7 * 100 = 20.72 %
So the oxygen volume will be 20.72%