## Ideal-Gas Equation

The ideal gas equation connecting pressure (P), volume (V) and absolute temperature (T) is PV = μ R T

since PV = constant and V/T = constant for a given quantity of gas, then PV/T should also be a constant. This relationship is known as ideal gas law or ideal-gas equation.

where μ is the number of moles and R is the universal gas constant.

R = 8.31 J mol^{-1 }K^{-1}

## Absolute Temperature

In the absolute temperature scale, the zero of the scale is the absolute zero of temperature – the temperature where every substance in nature has the least possible molecular activity. The Kelvin absolute temperature scale (T) has the same unit size as the Celsius scale (T_{C}), but differs in the origin:

T_{C} = T – 273.15

The size of the unit for Kelvin temperature is the same Celsius degree, so temperature on these scales are related by

T = T_{C} + 273.15