When a mixture of gases is placed in a container at a given temperature, the pre

Question

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 32g/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?

Explanation / Answer

Assuming the air to be made mainly of Oxygen and Nitrogen
Meffective = Mol frac of O2 * MW of O2+  Mol frac of N2 * MW of N2

Suppose Ratio of oxygen and nitrogen is x:(1-x)

X* 32 + (1-X)*28 = 29

X= 0.25

Hence wt. of Oxygen = 32*0.25 = 8 gm; wt. of nitrogen = 0.75*28 = 21 gm

Hence ratio of mass of oxygen to the mass of nitrogen in air = 8:21

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