Combined gas law

The combined gas law is a formula about ideal gases. It comes from putting together three different laws about the pressure, volume, and temperature of the gas. They explain what happens to two of the values of that gas while the third stays the same. The three laws are:

• Charles's law, which says that volume and temperature are directly proportional to each other as long as pressure stays the same.
• Boyle's law says that pressure and volume are inversely proportional to each other at the same temperature.
• Gay-Lussac's law says that temperature and pressure are directly proportional as long as the volume stays the same.

The combined gas law shows how the three variables are related to each other. It says that:

The formula of the combined gas law is:

${\displaystyle \qquad {\frac {PV}{T}}=k}$

where:

${\displaystyle P}$ is the pressure

${\displaystyle V}$ is the volume

${\displaystyle T}$ is the temperature measured in kelvin

${\displaystyle k}$ is a constant (with units of energy divided by temperature).

To compare the same gas with two of these cases, the law can be written as:

${\displaystyle \qquad {\frac {P_{1}V_{1}}{T_{1}}}={\frac {P_{2}V_{2}}{T_{2}}}}$

By adding Avogadro's law to the combined gas law, we get what is called the ideal gas law.