[jmv]

Hi,

I finished doing a problem and would like to know at what conditions does the # of molecules change if pressure and Temperature are held constant.

Reference Problem
A 0.100 L container maintained at constant temperature contains 5.0E10 molecules of an ideal gas.  How many molecules remain if the volume is changed to 0.005 L ?  What volume is occupied by 10,000 molecules at the initial temperature and pressure?

My answer
Since T is constant and only volume changed, the # of molecules at  volume 0.005L is the same as that of the initial volume.  Nothing is said about pressure, so I assume it's constant.

n = # of molecules(M)/avogadro's #(A)
V(final)=[10,000 molecules/5.0E10 molecules]*[V(inital)]
V(final)=2.E-8 L

[Borek]

Actually... number of molecules in a closed system stays the same, no matter whether you change temperature, volume or pressure. It is called mass conservation.

[Schrödinger]

A 0.100 L container maintained at constant temperature contains 5.0E10 molecules of an ideal gas.  How many molecules remain if the volume is changed to 0.005 L ? 

My answer
Since T is constant and only volume changed, the # of molecules at  volume 0.005L is the same as that of the initial volume.  Nothing is said about pressure, so I assume it's constant.

PV=nRT.
If n,R,T are constant and V is changed, how can P remain unchanged?

[Schrödinger]

Actually... number of molecules in a closed system stays the same, no matter whether you change temperature, volume or pressure. It is called mass conservation.

Well, the container is assumed to be closed. If not, the problem cannot be solved without assuming constant pressure, since no data on pressure change is available