 Now, so far we've been working out how many molecules there are in some object by counting the molecules. That's all very straightforward, but the trouble is you get very large numbers cut out, like 10 to the 27, 10 to the 28. And some people find these really big numbers a bit intimidating and hard to manipulate. And there's not even an SI prefix or something that big, the biggest SI prefix, the EXA, like an exometer or something that's only 10 to the 18, nothing like big enough to deal with these sort of numbers. So, some people prefer to invent a new unit called the mole. And one mole is 6.022 by 10 to the 23 molecules. This is called Avogadro's number. So if you want to work out how many molecules there are in something, the number is just equal to Avogadro's number times the number of moles. Now, that's not a profound physics equation, that's just a definition. Instead of counting molecules, 1, 2, 3, we count moles, one mole, two moles, three moles. And each mole is a whopping great number of molecules. And this is mostly done for historical accident. People knew about this long before they knew about the number of ideal atoms. But it is kind of handy in bringing the numbers down to more reasonable amounts. You'll find that the number of moles of molecules in something is probably 2 or 3 or 7 or 100 or something like that, rather than 10 to the 27. So it brings the numbers down to a more reasonable amount at the expense of introducing another constant. So, I personally don't think it's worth it. I'm happy to deal with big numbers. But some people would rather have smaller numbers and have an extra constant to remember your choice. Anyway, so if you know the number of moles of something, you can just multiply the Avogadro's number to work out the number of molecules. But how do you work out the number of moles of something? Well, you could just work out how many atoms there are of something, how many molecules, like we've talked about in the last two videos, and divide by Avogadro's number. But there's a shortcut. Now, the shortcut is that the number of moles is equal to the mass of something in grams divided by the molecular mass. Or if it's not a molecule, if it's just an atom, just divide it by the atomic mass. So that's actually pretty straightforward. So for example, a carbon, that's not a molecule, it's an atom, so use the atomic mass, which is 12. So 12 grams of carbon, it's 12 divided by 12, gives you one mole. Water, if you remember, had a molecular mass, it's H2O. O is 16, H is 1, and there are two of them, so that means 18. So that means 18 grams of water is one mole of water. So this is a convenient equation for working out the number of moles of something as long as you can work out the molecular mass.