At
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Solution: Click For PDF Version In the ideal gas approximation at the same temperature and pressure all the three gaseous substances air, oxygen and nitrogen will have the same number of molecules. Let this number be N. Let
be the mass of an oxygen molecule and
 be
the mass of a nitrogen molecule. Let V be the volume of the three
gaseous substances at
and at 1.00 atm
pressure containing N molecules. The densities of the oxygen gas and
nitrogen gas at
and at 1.00 atm
pressure are
 ,
and
 ,
respectively. Therefore,
Let the number
of oxygen and nitrogen molecules in air be
Substituting
we find
And
Therefore, fraction by mass of nitrogen in air is 0.73.
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,
,
and
,
respectively. As the density of air is
,
