# Heat Capacity of Air at Constant Volume

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?

Hootenanny
Staff Emeritus
Gold Member
What do you think it is?

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.

Andrew Mason
Homework Helper
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.

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

AM

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.

Is heat capacity independent of volume for an ideal gas?

Stupid question - gas performs work while being compressed.

Last edited:
mgb_phys
Homework Helper
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.

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?

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

Andrew Mason
Homework Helper
Hi,
This ratio of 1.4, does this just mean that you divide the constant pressure capacity (1.020J/g) by 1.4?
$\gamma = 1.4$ 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: