Volumes of Vapors

In addition to being very useful in calculating quantities for reactions (e.g. calculating how much sugar it takes to produce a certain quantity of alcohol in a fermentation), mols are extremely useful to distillers, because the number of molecules, not their size or weight, determines the volume of a vapor. This means that equal mols of vapor occupy the same volume. To be precise, one mol of a compound (in vapor form) occupies 22.4 liters at 0°C and one atmosphere of pressure, conditions defined as Standard Temperature and Pressure (STP). Most of the vapors you'll be dealing with will be at atmospheric pressure, but not at 0°C. Fortunately, you can calculate the volume they occupy at other temperatures and pressures by using the Ideal Gas Law. This law states that the pressure times the volume of a vapor, divided by it's temperature in degrees Kelvin (°K=° + 273) is a constant. The equation is written PV=nRT, where n is the number of mols and R is the Universal Gas Constant (8.3144 Joules, or 1.9872 calories per mol degree Kelvin). Using this equation, you find that one mol of vapor will occupy 30 litres at standard atmospheric pressure when the temperature is 92.625 °C (close enough to use 90 °C for practical purposes).

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