Question

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

Answer 1

Most gases closely follow an equation of state called the ideal gas law,
`P=(nRT)/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 J/(mol-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 (L-atm)/(mol-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=(nRT)/V = [(0.25mol)(0.082054 (L-atm)/(mol-K))(250K)]/(6L) = 0.855 atm`