How to find the Density of Air with PV=nRT?

[lc99]

Homework Statement

Let's say, P = 1.00 atm
and T =273K
and density of air at STP = 1.29 g/L

Homework Equations

The Attempt at a Solution

I'm not too sure...

PV = nRT
n = PV/RT
= P/RT = 1/(0.0821*273) *1.29 g/L ?

 

[Borek]

PV=nRT alone is not enough, you need air composition as well.

For any gas: assume 1 m³ of gas, can you calculate number of moles? Mass? That will give you density instantly.

For air: air has no molar mass, as it is a mixture, but every mixture has its apparent molar mass (behaves as a gas of such), one that can be calculated as a weighted average of molar masses of the components.

Alternatively: assume 1 m³ of gas. Knowing the air composition, can you calculate volume occupied by the nitrogen? Or its partial pressure? Of other gases?

 

[lc99]

PV=nRT alone is not enough, you need air composition as well.

For any gas: assume 1 m³ of gas, can you calculate number of moles? Mass? That will give you density instantly.

For air: air has no molar mass, as it is a mixture, but every mixture has its apparent molar mass (behaves as a gas of such), one that can be calculated as a weighted average of molar masses of the components.

Alternatively: assume 1 m³ of gas. Knowing the air composition, can you calculate volume occupied by the nitrogen? Or its partial pressure? Of other gases?

im sort of confused because i was told to use the ideal gas law. the question is part of a lab I am doing and I am suppose to find the density of the air based on the lab's temperature and volume.

 

[Borek]

I gave you plenty of hints, have you tried to use them?

 

[lc99]

i

I gave you plenty of hints, have you tried to use them?

figured out how to get the moles of air.. so, the weighted avg of molar mass would be the a lot of gases wouldn't it?

i feel like I am missing some information. would finding the molar mass from the density of air at stp help to find molar mass of air?

so M = dRT/P at stp = 28.913 g/mol

so i can use the equation d = PM/RT for any temperature and pressure?

 

[Chestermiller]

i

figured out how to get the moles of air.. so, the weighted avg of molar mass would be the a lot of gases wouldn't it?

i feel like I am missing some information. would finding the molar mass from the density of air at stp help to find molar mass of air?

so M = dRT/P at stp = 28.913 g/mol

so i can use the equation d = PM/RT for any temperature and pressure?

You can use this equation at any temperature and pressure at which the ideal gas law is a good approximation for your particular gas.

 

[Borek]

the weighted avg of molar mass would be the a lot of gases wouldn't it?

Nope, enough to treat the air as a mixture of just nitrogen and oxygen, other ones are in minute quantities and can be safely ignored.

 

[lc99]

Nope, enough to treat the air as a mixture of just nitrogen and oxygen, other ones are in minute quantities and can be safely ignored.

Thanks for the help!