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Equal masses of methane and oxygen are mixed in an empty container at 25°C. The fraction of the total pressure exerted by oxygen is

1. 2/3
2. 1/3
3. 1/2
4. 1/3 × 273/298

Correct answer: 1/3

Solution

For equal mass m, moles of CH4 = m/16 and O2 = m/32. Mole fraction of O2 = (1/32)/(1/16+1/32) = (1/32)/(3/32) = 1/3, so oxygen exerts 1/3 of the total pressure.

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