MathBench > Measurement

Calculating Molar Weight

Molar weights made easy

The guy who came up with 602,200,000,000,000,000,000,000 (named Avogadro) probably didn't like all those zero's any more than you do. So he did something very smart: he defined a mol as

the number of carbon atoms in 12 grams

It turns out 602,200,000,000,000,000,000,000 is approximately that number.


carbon atomBut why 12 grams? Well, if you remember something about atomic structure, you know that a carbon atom has six protons and six neutrons (which means it has 6 electrons as well, 4 of which are on its outer shell, allowing it to form 4 covalent bonds and be backbone of organic life ... but that's a different story)

Anyway, back to the 12 grams. Since a proton and a neutron each weigh 1 atomic unit, a carbon atom weighs about 12 atomic units (the electrons don't weigh much of anything). And a mol of carbon weighs 12 grams. That means 1 mol of protons or neutrons weighs 1 gram.