Finding the Volume of an Ideal Gas

[Kajan thana]

Homework Statement

A rigid tank contains 5 kg of air at 100 kPa and 27 °C. The air is now heated until
its pressure doubles. Determine the volume of the air?

Relevant Equations

1) PV= mRT.
2) Denisty = Mass/Volume

Hi,

I tried to do this question in two different approaches one of them was using the equation PV=mRT where I got the right answer which is 4.305 m**2. However, I tried using this Density = Mass/Volume, where I substituted Denisity= 1.225 and Mass equals 5kg to get the volume as 4.08.
Can someone tell me why I can't use the second equation? Is it because the density of the air varies?

 

[jbriggs444]

Is it because the density of the air varies?

Yes. The density of air will vary with temperature and pressure. You are given values for both and should use those values.

The volume of the air is most definitely not 4.305 m². Can you see why?

 

[Kajan thana]

Yes. The density of air will vary with temperature and pressure. You are given values for both and should use those values.

The volume of the air is most definitely not 4.305 m². Can you see why?

Thank you.
It makes sense that the density will vary with temperature and pressure so I have to use the values that is given. But can please explain why 4.305m² is not also the right answer ( according to marking scheme, it is ) ?

 

[mjc123]

Hint: the problem is not with the number, but the units.

 

[Kajan thana]

Hint: the problem is not with the number, but the units.

Oh thank you.. Yh wrong unit..