# What is the formula for calculating the pressure of a gas?

Nov 5, 2016

Most gases closely follow an equation of state called the ideal gas law,
$P = \frac{n R T}{V}$.

#### Explanation:

It's important to pay attention to the units, however. In strict SI units (highly recommended), express n in moles, R is the universal gas constant $R = 8.314 \frac{J}{m o l - K}$, T is the temperature in Kelvins, and the volume V is in ${m}^{3}$. The resulting pressure P will be in Pa.

The other common set of units is where V is in liters (L) and
$R = 0.082054 \frac{L - a t m}{m o l - K}$, in which case the pressure is calculated in atm.

Example:
0.25 mol of helium gas confined to a 6L vessel at 250K will have a pressure equal to
$P = \frac{n R T}{V} = \frac{\left(0.25 m o l\right) \left(0.082054 \frac{L - a t m}{m o l - K}\right) \left(250 K\right)}{6 L} = 0.855 a t m$