Moles and gases

Since many chemical substances exist as gases, it is often much more practical to find how many moles of a gas are present by measuring its VOLUME rather than by weighing out its mass. Building on Avogadro's law, chemists have established that one mole of ANY gas — regardless of which gas it is — occupies a fixed volume of 22.4 dm3 at standard temperature (0°C) and pressure (1 atm), a condition abbreviated STP; this fixed volume of 22.4 dm3 at STP is called the molar volume of a gas. Using this, the number of moles of a gas sample can be found directly as: number of moles (n) = volume of the gas at STP divided by the molar volume of a gas (22.4 dm3 mol-1); and once the number of moles is known, the number of molecules follows immediately as number of moles multiplied by Avogadro's constant (6.022 x 10^23 molecules mol-1). It is worth noting that IUPAC has, more recently, revised the standard pressure used to define STP from 1 atmosphere to 1 bar; under these new …