The ideal gas law relates several factors of a gas, and one of these factors is the gas's quantity, measured in moles. The pressure and volume of a gas depend on the number of moles of molecules in it. The more molecules in a gas, the more space it must take or the more pressure it must contain. A single mole of a gas takes up 22.4 liters at standard temperature and pressure. Use any gas's volume and conditions to calculate the number of moles that it contains.

Divide the gas's volume by 22.4. If the gas takes up a volume of, for instance, 2 liters, 2 divided by 22.4 will equal 0.08928. If this gas is at standard temperature and pressure, it contains 0.08928 moles.

Divide the gas's pressure by standard pressure, which is 101,000 Pascals. If its pressure is, for instance, 50,000 Pascals, 50,000 divided by 101,000 will equal 0.495.

Divide the answer from Step 1 by the answer to Step 2  0.08928 divided by 0.495 will equal 0.18036. The gas contains 0.18036 moles if it is at standard temperature.

Divide standard temperature, which is 273.15, by the gas's temperature in Kelvin. If its temperature is, for instance, 400 K, 273.15 divided by 400 will equal 0.6829.

Multiply together the answers to Step 3 and Step 4: 0.18036 x 0.6829 = 0.1232. This is the number of moles of molecules in the gas.
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