Knowing this scale makes relative measurements easy, because 18.0 grams of water has the same number of molecules as 46.1 grams of ethanol. Such quantities, assigning a practical unit of measurement instead of using "atomic units" is called a mole, and if using grams then 18.0 grams of water would be 1 gram mole, and 18.0 kilograms of water would be 1 kilogram mole. Of course, a kilogram mole of water would have 1000 times as many molecules as a gram mole, and if you used pounds for your measurements, then a "pound mole" would have yet another different number of molecules. The advantage of this system is that if you stick to any one practical unit of measurement, then 1 "whatever-it-is" mole of one substance will have exactly the same number of molecules as 1 "whatever-it-is" mole of another substance. If you measured out 18.0 pounds of water and 46.1 pounds of ethanol, you would have the same number of molecules of each compound. The unit of measurement used most frequently in chemistry is the gram. To save time, a "gram mole" is often referred to simply as a "mol", and this is what we'll mean from now on. Chemical engineers often use the kilogram mole, and this is distinguished by having a capital letter, Mol.

You must be very careful to use the correct molecular formula when calculating mols. For example, oxygen atoms have a mass of 16 atomic mass units, but 16 grams of oxygen is not a mol of oxygen gas, it's only half a mol. This is because oxygen normally exists in molecules containing of two oxygen atoms bound together, with a combined mass of 32 atomic mass units. So one mol of oxygen gas has a mass of 32 grams.

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