The ideal gas law comes from Kinetic-Molecular Theory (KMT),
and also from the fact that PV and nRT both have the units 
, which are the same
units as energy. PV is energy in the form of work that the gas can do on its
surroundings, and nT is the internal energy that the gas molecules have because
of their motion. (R is the constant that makes the two sides numerically
equal.) This means that the ideal gas law is just the law of conservation of
energy—the internal energy of the molecules equals the work that the gas is
capable of doing on its surroundings.
The Gas Constant (R):
Because R is just a number that makes the energy balance work, its value depends on the units that you are using:
