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Question

Air contains 23% of oxygen and 77%of nitrogen by weight. What is the percentage of oxygen by volume is?


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Solution

Step 1: Given data

Percentage of oxygen=23%

Percentage of nitrogen=77%

Step 2: Calculating the percentage of oxygen

Let the mass of air=100g

Therefore, the mass of oxygen in 100g of air=23g

and mass of nitrogen in 100g of air=77g

Also, the molecular mass of oxygen=32g

and the molecular mass of nitrogen=28g

Now, the number of moles of oxygen=givenmassmolecularmass22g32g

and number of moles of nitrogen=givenmassmolecularmass77g28g

As one mole of gas requires 22.4L of volume at Standard Temperature and Pressure (STP) conditions, So

The volume occupied by moles of oxygen=23g32g×22.4L16.1L

The volume occupied moles of nitrogen=77g28g×22.4L61.6L

Therefore, the total volume of air=volume occupied by moles of oxygen+volume occupied by moles of nitrogen

=16.6L+61.6L77.7L

So percentage by volume of oxygen=volumeoccupiedbymolesofoxygentotalvolumeofair×100

=16.177.7×10020.7%

Therefore, the percentage of oxygen by volume is 20.7%


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