Most chemical measurements are made in grams. An amount of any substance in grams that is numerically equal to its atomic or molecular weight in amu has been defined as one mole of that substance.

By this definition, one mole of hydrogen is 2.016 grams, one mole of methane is 16.043 grams, and one mole of water is 18.015 grams. We can convert any gram quantity of a chemical substance to moles by dividing by its molecular weight. Once we have done this, we know that equal numbers of moles of all kinds of substances must have equal numbers of molecules. The same number of molecules is present in a mole of hydrogen, water, methane, or any other substance. This is very useful, because then we can measure the right amounts of starting material for chemical reactions, and can tell from the results how many molecules of product were formed per molecule of reactants.

Example. How many moles of carbon are present in the 100 g of the preceding example? How many moles of hydrogen atoms would be needed to combine with these? How many grams of hydrogen would be needed?

Solution. The number of moles of carbon is

(100g carbon / 12.011 g ) = 8.33 moles of carbon

Four times as many hydrogen atoms are needed as carbon atoms to make methane, , so four times as many moles will be required also:

(4 moles H / 1 mole C) x 8.33 moles C = 33.3 moles of hydrogen

Since the atomic weight of hydrogen is 1.008, this corresponds to

33.3 moles hydrogen x 1.008 g = 33.6g of hydrogen

This is the same answer as we obtained previously, but this time we used moles instead of merely the ratio of atomic weights.