# In the Ideal Gas Law, what does “fixed amount” of gas mean?

Does it mean mass? If so, wouldn't Pressure/Temperature basically represent its density?

• $PV=nRT$ only does not take into account the mass the amount that is referred to is the number of moles, a quantitative measure that states the number of particles – Jaywalker Apr 21 '15 at 18:14
• Why would Pressure/Temperature represent density? – Shubham Apr 21 '15 at 20:10
• @Shubham Well, it might be naive, but the reason I thought it might represent density is because mass= density*volume. – HyperLuminal Apr 22 '15 at 11:30

## 2 Answers

Sounds like constant amount of substance (as in the dimension or physical quantity, "mol" being it's unit). Could also mean mass, but mass is often not strictly constant. If you add energy (e.g. heat), the mass of the gas increases slightly via E=m*c2.

Could you provide more context?

• According to what I read, "for a fixed amount of gas, the pressure, temperature, and volume varies accordingly:" Then it gives the equation. Mol actually sounds like a very likely unit, come to think about it; as it refers to the number of atoms. – HyperLuminal Apr 22 '15 at 11:28

In thermodynamics physicists and engineers use the concept of 'control volume' that specifies a bounded region of space in which thermodynamic properties of a gas are analyzed.

The boundary of the control volume can be used to either isolate or otherwise define a specific flux of energy and/or matter to/from the control volume.

I believe the "fixed amount" of gas refers to a situation where the control volume has zero mass flux through the boundary, but not necessarily zero energy flux.

• Darsie said the unit was "mol." What do you think? – HyperLuminal Apr 22 '15 at 11:29