According to Boyle's gas law, a gas's pressure and volume are inversely proportional at a fixed mass. Halving a gas's volume, for instance, doubles the gas's pressure if other conditions are constant. Physicist Robert Boyle first published the relationship in 1662 after measuring the expansion of gas through a J-shaped glass tube. To calculate a volume of gas's air pressure, you must also know the other factors in the ideal gas law. These factors include the gas's temperature and the number of molecules within it.
Divide the number of moles of gas by the gas's volume, measured in cubic meters. If the gas contains, for instance, 200 moles of gas, and it takes up 10 cubic meters -- 200 / 10 = 20.
Multiply the answer by the gas's temperature, measured in Kelvin. If the gas is at, for instance, 250 Kelvin -- 20 x 250 = 5,000.
Multiply the answer by the universal gas constant, which is 8.3145 joules per moles Kelvin -- 5,000 x 8.3145 = 41,572.5, or about 41,500. This is the gas's pressure, measured in Pascals.
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