When a mixture of gases is placed in a container at a given temperature, the pressure in the container is equal to the sum of the pressures that the individual gases would exert if they separately occupied the entire container at the same temperature. Given that air is basically a mixture of nitrogen, with a molecular weight of 28 g/mol, and oxygen, with a molecular weight of 32 g/mol, what must be the ratio of the mass of oxygen to the mass of nitrogen in air for air to have an effective molecular weight of 29?
A) 8:21
B) 3:5
C) 5:7
D) 32:87
E) 3:7

Question

Quizplus · Accepted Answer

To find the ratio of the mass of oxygen to nitrogen, use the formula for the effective molecular weight: $ M = xN_2 \cdot 28 + xO_2 \cdot 32 $, where $ xN_2 $ and $ xO_2 $ are the mole fractions. Solving for the given effective molecular weight of 29, the ratio of the mass of oxygen to nitrogen is 8:21.

When a Mixture of Gases Is Placed in a Container