The ideal gas law, is an equation of state for an ideal gas. It combines three gas laws (Dalton's Law, Boyle's Law and Charles' Law) into one equation:
- PV = nRT
where
- P is the pressure of gas;
- V the volume the gas occupies;
- T the absolute temperature (meaning it must be in Kelvin or Rankine);
- n is the number of moles of the gas occupying the volume V;
- R is the gas constant.
The gas constant can be expressed in any number of units, but the most common representations are 0.0821 
or 8.314 
Ideal Gas
The equation is valid only for an ideal gas, the hypothetically perfect embodiment of a gas in which the particles (atoms or molecules) in the gas are spherical, identical, have no volume, and experience no intermolecular forces between them. All collisions between the particles or the particles and the container are perfectly elastic.
Since this is just a model, real gases only obey the ideal gas law approximately, not perfectly. Generally, the ideal gas assumption is accurate for unreactive gases at high temperature and/or low pressure. A good rule of thumb is that the assumption can be used above room temperature and below 1 atmosphere of pressure.