Example. A hardware store clerk is told to weigh one pound of machine bolts for a customer, and also to weigh approximately enough hexagonal nuts to go with them. He finds that a hex nut weighs 0.40 as much as a machine bolt of the type requested. How many nuts should he include with the order?
Answer. He should include 0.40 X 1 pound = 0.40 pound of hex nuts
Such a procedure would be good enough for most real situations, and a lot easier and faster than sitting down and counting individual pieces. This is exactly what the chemist does with molecules.

Example. A chemist wants to make as much methane, , as he can from 100 g of carbon. How much hydrogen will be required?
Solution. The atomic weight of hydrogen is 1.008 amu, and that of carbon is 12.011 amu. Four hydrogen atoms are required for each carbon atom, so 4x1.008 amu = 4.032 amu of hydrogen will be needed for each 12.011 amu of carbon. The relative weights of hydrogen and carbon will be 4.032 units of hydrogen to 12.011 units of carbon, whatever the weighing units chosen. The problem was expressed in grams. Thus

100g carbon x (4.032g hydrogen / 12.011g carbon) = 33.6g hydrogen

will be required. Like the hardware store clerk, the chemist can weigh 100g of carbon and 33.6g of hydrogen and assume that he has the right relative number of atoms without counting them.