[zerality]

Hi

I need assistance with this problem:
what is the density of air at 25 degrees centigrade and 745 torr if the composition by mole fraction is 78% nitrogen, 21 % oxygen and 1% argon.

I know that density = ( molar mass x pressure (atm))/ (RT)

Do i find the partial pressures of each gas and use that to find the density of each gas and then sum the densities?

[enahs]

I know that density = ( molar mass x pressure (atm))/ (RT)

Do i find the partial pressures of each gas and use that to find the density of each gas and then sum the densities?

The easiest and most direct route would be to calculate the average molar mass of air with the numbers provided, and then use that in your equation of density for a gas.

Just make sure to remember which elements are diatomic and to keep track of your units and use the appropriate Idea Gas Constant so your units cancel properly, and you know what unit you are left with.

[zerality]

For the average molar mass of air, do I just sum the masses of 02, N2 and Ar; or do I multiply the individual masses by the percentage of each gas and then sum the total?

[Borek]

Multiply by percentages, you need weighted average.