Method We expect species with a high mole fraction to have a proportionally high
partial pressure. Partial pressures are defined by eqn 1.13. To use the equation, we
need the mole fractions of the components. To calculate mole fractions, which are
defined by eqn 1.14, we use the fact that the amount of molecules J of molar mass
MJ in a sample of mass mJ is nJ = mJ/MJ. The mole fractions are independent of the
total mass of the sample, so we can choose the latter to be exactly 100 g (which
makes the conversion from mass percentages very easy). Thus, the mass of N2 present
is 75.5 per cent of 100 g, which is 75.5 g.
Method We expect species with a high mole fraction to have a proportionally highpartial pressure. Partial pressures are defined by eqn 1.13. To use the equation, weneed the mole fractions of the components. To calculate mole fractions, which aredefined by eqn 1.14, we use the fact that the amount of molecules J of molar massMJ in a sample of mass mJ is nJ = mJ/MJ. The mole fractions are independent of thetotal mass of the sample, so we can choose the latter to be exactly 100 g (whichmakes the conversion from mass percentages very easy). Thus, the mass of N2 presentis 75.5 per cent of 100 g, which is 75.5 g.
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