- pV = nRT
Here p is the pressure of gas, V the volume it occupies, n the number of moles of gas, R the molar gas constant, and T the temperature of the gas.
The equation is valid only for the ideal gas. Real gases obey this equation only approximately, but its validity increases as the density of the gas tends to zero.
The picture shows some isotherms of the ideal gas (i.e. the relation between pressure p and volume V at fixed temperature T; plotted for a set of temperatures, with increasing T from lower to upper curve):
See also: