How to Find the Moles of a Gas


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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