 Using moles. You should already have met Avogadro's number 6.02 x 10 to the 23, which describes what is known as one mole or 12 grams of carbon atoms. In this video we are going to learn how to use moles to do some chemistry calculations. For any element the relative atomic mass or AR is the weight in grams for one mole. When we compare chemicals we compare equal numbers of particles even though the weights are different. So we have compounds instead of atoms it doesn't matter. So if we have compounds instead of atoms it doesn't matter how many atoms are in the formula only the number of compound particles. Let's say you have one mole of iron atoms but they are in a compound FeCl3. This means you also have one mole of the compound. The ratio of iron atoms to particles of compounds is one to one. What is the ratio of chlorine atoms to particles of the compound? Pause the video whilst you think. Three to one. There are three chlorine atoms per formula unit. For any compound the relative molecular mass or MR is the weight in grams for one mole or 6 x 10 to the 23 particles. What if you had some Fe2O3 instead? For one mole of iron you would have half a mole of the compound because there are two moles of iron for every one particle of the compound. One divided by two is one half. You can have half moles of anything or quarter moles or hundredths of moles because a mole is a really big number, 6.02 x 10 to 23. 6.02 x 10 to the 23 can be broken down into smaller numbers so it's okay to have fractions of moles. You can also double them to say that for two moles of iron you have one mole of compound. This is okay too because the numbers are relative. How many moles of oxygen would you have in Fe2O3? Pause the video and think about it carefully. The answer is three if you are using one mole.