In 1811 Avogadro noted the relationship between volume and amount of a gas.
This is consistent with a balloon increasing in volume when gas is added to it.
Because n (amount in moles) is a measure of the number of particles of gas present,
the same volume of different gases at the same
P and
T contains the same number of particles.
One mole of most common gases occupies 22.4
at 0°C and 101.3 kPa (1 atm).
| PV | = n × constant |
| T |
| PV = nRT |
The relationship between volume and amount can be combined with Boyle's and Charles Law to give the relationship shown at the right.
This is referred to as the ideal gas law.
If P is in Pa, V is in m3 and T is in K
the constant R is 8.314 J mol–1 K–1.
This R appears in many other relationships.
R has a different value if V is in L and P is in atm.
Recall that the temperature in Kelvin is equal to the temperature in Celsius plus 273.15.
Ideal gases are those that obey this law under all conditions. The particles in ideal gases
are point masses (no volume)
continually move in straight lines.
collide elastically (total kinetic energy is constant) and change in direction as a consequence of collision.
are not attracted to one another
Real gases (nitrogen, oxygen, carbon dioxide....) are not ideal because their particles have volume and are attracted to one another. Therefore real gases obey the ideal gas law most closely if their pressure is lower than 200 kPa.