An ideal gas is defined as a gas in which the molecules can be considered as "point masses" thereby taking up negligible volume compared to the volume of the container. No significant forces, intermolecular interactions, exist between the molecules except during collisions which are considered to be perfectly elastic. All real gases that are far removed from conditions at which they condense, low temperatures or high pressures, display nearly ideal behavior.

 

Avagadro's Number:  N_A = 6.022 x 10²³

 

The number of carbon atoms in 12 grams of a sample of carbon-12 is called Avogadro's number, N_A, which equals 6.022 x 10²³ molecules. A mole of any substance contains Avagadro's number of particles.

 

 

Ideal Gas Law: PV = nRT

 

where

 

Combined Gas Law:

where

 

 

When selected quantities are held constant then this law can be restated as three others.

 

Boyle's Law	temperature and the amount of gas present held constant
Charles' Law	pressure and the amount of gas present held constant
Guy-Lussac's Law	volume and the amount of gas present held constant

 

Some fundamental relationships

 

Often in determining the amount of gas present in moles (n), you need be familiar with the following relationships between Avogadro's number (N_A), the amount of mass present (m), molecular mass (N), and the mass of a single molecule ().

 

 

    N = nN_A

 

This relationship let's us rewrite the ideal gas law as:

PV = (N/N_A) RT
PV = N (R/N_A) T
PV = Nk_BT where k_B is Boltzmann's constant, 1.38 x 10^-23 J/K.

 

 

    m = nM

 

 

    m = N

 

 

    M = N_A   

 

 

Now let's take some time and practice several problems that will allow you to become familiar with these formulas.