Gas Laws - Combined and Ideal Gas Laws

Combined and Ideal Gas Laws

The combined gas law or general gas equation is formed by the combination of the three laws, and shows the relationship between the pressure, volume, and temperature for a fixed mass of gas:

With the addition of Avogadro's law, the combined gas law develops into the ideal gas law:

where

P is pressure

V is volume

n is the number of moles

R is the universal gas constant

T is temperature (K)

where the constant, now named R, is the gas constant with a value of .08206 (atm∙L)/(mol∙K). An equivalent formulation of this law is:

where

P is the absolute pressure

V is the volume

N is the number of gas molecules

k is the Boltzmann constant (1.381×10−23 J·K−1 in SI units)

T is the temperature (K)

These equations are exact only for an ideal gas, which neglects various intermolecular effects (see real gas). However, the ideal gas law is a good approximation for most gases under moderate pressure and temperature.

This law has the following important consequences:

1. If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas.
2. If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present.
3. If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume.
4. If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change in direct proportion to the temperature.