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Chemistry Question 51 – JEE-MAIN 2025

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At the sea level, the dry air mass percentage composition is given as nitrogen gas: 70.0, oxygen gas: 27.0 and argon gas: 3.0. If total pressure is 1.15 atm, then calculate the ratio of following respectively: (i) partial pressure of nitrogen gas to partial pressure of oxygen gas (ii) partial pressure of oxygen gas to partial pressure of argon gas (Given: Molar mass of N, O and Ar are 14, 16 and 40 g mol−1 respectively.)

The partial pressure of a gas in a mixture is directly proportional to its mole fraction.

Step 1: Convert mass percentages to moles for each gas✦ Active

Assume a total mass of 100 g for the dry air mixture. The molar masses are MN2=2×14=28 g/mol, MO2=2×16=32 g/mol, and MAr=40 g/mol. The moles of each gas are calculated as:

nN2=70.0 g28 g/mol=2.5 mol nO2=27.0 g32 g/mol=0.84375 mol nAr=3.0 g40 g/mol=0.075 mol
Step 2: Calculate the ratio of partial pressure of nitrogen gas to partial pressure of oxygen gas○ Expand

According to Dalton's Law of Partial Pressures, the ratio of partial pressures is equal to the ratio of their moles.

PN2PO2=nN2nO2=2.50.84375≈2.96
Step 3: Calculate the ratio of partial pressure of oxygen gas to partial pressure of argon gas○ Expand

Similarly, the ratio of partial pressures of oxygen to argon is:

PO2PAr=nO2nAr=0.843750.075=11.25

The calculated ratios are 2.96 and 11.25, which corresponds to option 2.

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