# Homework Help: Kinetic theory: gas/pressure

1. Jun 20, 2011

### ajmCane22

1. The problem statement, all variables and given/known data

An ideal gas is kept in a container of constant volume. The pressure of the gas is also kept constant
(a) If the number of molecules in the gas is doubled, does the rms speed increase, decrease, or stay the same?
(b) If the initial rms speed is 1300 , what is the final rms speed?

2. Relevant equations

PV=nRT...I think

3. The attempt at a solution
For part I, I believe the rms speed decreases. So if the only thing changing is the number of molecules wouldn't doubling the molecules cause rms to decrease by half? I know the answer is no, because the computer said so, I'm just not sure why it's wrong. And how much does it decrease then?

2. Jun 20, 2011

### willem2

If the volume and the pressure must remain constant, the only way to get in more molecules is cooling the gas, and let in more gas to keep the pressure constant.

If the original tempererature was T, what is the new temperature?

After you've found it use

$$v_{rms} = \sqrt { \frac {3 R T} {m} }$$

where m is the mass of one mole of the gas molecule