Scientists use the ideal gas law to provide approximate properties of gases. The gas law states that PV = nRT, where P represents the gas’s pressure, V represents its volume, n represents the moles of gas, R represents the ideal gas constant of 0.08206 liter atmospheres per mole per Kelvin and T represents the temperature in Kelvin. Therefore, to convert the moles of gas to pressure, the scientist must know the volume and temperature of the gas, in addition to the number of moles of gas. The pressure is then given by P = nRT / V.
Convert the volume and temperature to units of liters and Kelvin, respectively, if necessary. Numerous online conversion utilities are available to help you with this task, such as that provided in the Resources section. For example, a volume of 22 cubic feet of gas at 78 degrees Fahrenheit converts to 623 liters at 299 Kelvin.
Calculate the pressure of the gas, P, in units of atmospheres according to P = nRT / V. For example, if the sample of gas occupying 623 liters at 299 Kelvin represents 55 moles of gas, then P = (55 x 0.08206 x 299)/623 = 2.17 atmospheres.
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Convert the pressure to your preferred units, again by using an online calculator. A pressure of 2.17 atmospheres, for example, converts to 220 kilopascals, 31.9 pounds per square inch or 64.9 inches of mercury.