# Ideal gas law problems: Pascals vs Atms?

1. Jan 15, 2005

### ddoctor

Hi group,

Simple arithmetic problems using PV = nRT or NkT. How do you know when to use pressure units in atmospheres or pascals? For example, in 1 particular problem they give you a pressure in units of Torr, volume in cubic cm and temp in Kelvin and ask for the number of molecules. Couldn't you convert Torr to atm, cm to L OR Torr to Pascals and cm to m. In the first case you get n number of moles and multiply by avagadro's number to get molecules. In the second case you use NkT to get the number of molecules directly, probably easier, but aren't both approaches correct?
Thanks
Dave

2. Jan 15, 2005

### Integral

Staff Emeritus
As long as all of your units are in the same system and you use the appropriate constants (in the same units) you will be fine. Just keep track of your units to ensure that your answers make sense.

3. Jan 15, 2005

### dextercioby

My advice is:use SI-mKgs.The most correct and reliable.Sometimes involves big numbers,but,hey,where would the fun be in such problems??

Daniel.

4. Jan 15, 2005

### Gokul43201

Staff Emeritus
When using SI units : P is in Pa (or N/m^2), V is in m^3, T is in Kelvin and R = 8.314 J/K-mol

When using the other units : P is in atm, V is in Liters, T is in Kelvin and R = 0.0821 L-atm/K-mol

You can you any system you like. Remember to use the right value of R in that system.

If you are given values in units other than the two above (eg : torr, psi, ml, cc, centigrade, fahrenheit), you can simply convert to either one system of units.