GASES
The volume occupied by a gas depends on a number of factors:
 the temperature: the hotter the gas, the faster the particles are moving and the more space they will occupy
 the pressure: the higher the pressure, the more compressed the gas will be and the less space it will occupy
 the amount of gas: the more gas particles there are, the more space they will occupy
The volume occupied by a gas does not depend on what gas it is, however: one mole of any gas, at the same temperature and pressure, will have the same volume as one mole of any other gas.
Vm is the molar volume which is equal to 22.7 dm3 at 273K and 100kPa.
The pressure, temperature, volume and amount of gas can be related by a simple equation known as the ideal gas equation:
PV = nRT

P is the pressure measured in pascals (Pa) or Nm^{2}. One atmosphere, which is normal atmospheric pressure, is 101325 Pa.
V is the volume in m^{3}. Remember; 1 m^{3} = 1000 dm^{3} = 10^{6} cm^{3}.
T is the absolute temperature, measured in Kelvin (K). Remember; 0 ^{o}C = 273 K.
R is the molar gas constant and has a value of 8.31 Jmol^{1}K^{1}.
This equation can be rearranged to find the density of gases and the RMM of gases, using the relationship m = n x m_{r}.
PV = mRT/m_{r}, so the mass of one mole is given by m_{r} = mRT/PV, where m is the mass in kg. The answer m will also be in kg so it must be converted into grams.
The density of a gas, or mass/volume, is given by (m/V) = m_{r}P/RT.