# Heat Capacity of Air at Constant Volume

1. ### s.p.q.r

25
Hi

I have an ongoing dispute with my mate on this one, please help to clarify this before I open up a can of whoop ass on that sorry mo-fo.

300 litres of air are compressed into a 3 litre tank. What is the heat capacity of this air?

2. ### Hootenanny

9,678
Staff Emeritus
What do you think it is?

3. ### s.p.q.r

25
The Cp J mol is 29.19. But because I ask for constant volume, it is definately lower then this. This is what I think. I can find no references to constant volume anywhere and unfortunately I have no teacher to ask as I study archaeology, not physics.

4. ### Andrew Mason

6,859
Air is almost entirely a diatomic gas, $\gamma = C_p/C_v = 1.4$ (7/5)

AM

5. ### s.p.q.r

25
Hi,

Thanks for the reply. Is 1.4 per gram or mol?

Also,

How can you measure a gram of gas and how much is 1 mol?

Cheers.

8,952
7. ### Loren Booda

Is heat capacity independent of volume for an ideal gas?

Stupid question - gas performs work while being compressed.

Last edited: Oct 2, 2007
8. ### mgb_phys

8,952
For an ideal gas heat capcity just depends on the amount (number of moles) present and the number of vibration states of the molecular.
For a real gas it also depends on the pressure because the molecules close to each other change the vibration state/bond energy.

9. ### Loren Booda

In a modification of the "ideal gas" law, I seem to recall an equation with correction terms for the volume and pressure, respectively. Has anyone run across this?

10. ### s.p.q.r

25
Hi,
This ratio of 1.4, does this just mean that you divide the constant pressure capacity (1.020J/g) by 1.4?

11. ### Andrew Mason

6,859
$\gamma = 1.4$ is the ratio of the specific heat (heat flow per gram or per mole per degree K change in temperature) at constant pressure to the specific heat at constant volume. $\gamma = C_p/C_v$. What you want to find is Cv. You also have to find the number of moles of air in this container to find its heat capacity (heat flow per degree K change in Temp.).

AM

Last edited: Oct 8, 2007