The Ideal Gas equation

The pressure law (P/T = constant) helps us find out what happens to the pressure and temperature of a gas when one or other of these quantities changes.
Charles' law (V/T = constant) helps us find out what happens to its volume and temperature when either one of these quantities changes and finally Boyle's Law (PV = constant) helps us find out what happens to the pressure and volume of a gas when one or other of these quantities changes.

If we combine the two equations we get another equation involving all three – pressure, volume and temperature.

This equation is called the gas equation or more properly the Ideal Gas Equation.




The quantity R is a constant called the universal gas constant and its value is 8.3 J/mol K.
Remember that in all these gas calculations the temperature must be in Kelvin (^oC + 273)

Standard temperature and pressure

In some chemical experiments you need to know the volume that a gas would occupy at 0^°C and 760 mm Hg pressure. These values of temperature and pressure are known as standard temperature and pressure or STP.
The pressure exerted by a 760 mm high column of mercury is 101 396 Pa. This is often called one atmosphere and simplified to 100 000 Pa.
One useful fact is that one mole of any gas at STP occupies a volume of 22.4 litres or 22.4x10^-3 m³.