If I wanted to know how much a dozen eggs weighed, I could take the average weight for one egg and multiply by 12:

50 grams x 12 = 600 grams

That was easy. We could do the same thing with atoms – for example, a mol of helium:

.0000000000000000000000664 g x 602,200,000,000,000,000,000,000 = ???

Two problems here:

•  How did I know how much a helium atom weighs?

•  What a nasty computation!!! Miss a few zeros and you’re completely screwed. There must be an easier way…