In this video, we're going to cover what the term mole means and see how we can use this formula to convert between moles mass and relative formula mass. Now, the term mole is a bit weird, but really it's just a unit we use to measure the amount of chemical that we have. So just like we measure distance in meters and time in seconds, we can measure how much substance we have in moles. And one mole of any substance is just the amount of that substance that contains 6.02 * 10 23 particles with particles referring to atoms, molecules, ions, or even electrons depending on which substance you're talking about. So if we had a little pile of carbon and we were told that it contained exactly one mole of carbon then there must be 6.02 * 10 23 atoms of carbon in that pile. We call this number Avagadro's constant. And the reason it's this specific number is that the mass of that many particles of any substance will be exactly the same number as that substance's relative atomic or formula mass in g. So our little pile of 1 mole of carbon would weigh exactly 12 g because carbon's relative atomic mass is 12. Meanwhile, one mole of oxygen which has the relative formula mass of 16 * 2. So 32 would weigh 32 g or 1 mole of CO2 with a relative formula mass of 12 + 16 + 16 and so 44 would weigh 44 g. But in all these cases there would be 6.02 * 10 23 atoms or molecules. Because of this rule, we can create a formula which tells us that the number of moles in a sample is equal to the mass of that element or compound divided by its mr. So if we want to know how many moles there were in 42.5 g of ammonia, we would do 42.5, which is the mass divided by 14 + 3 * 1, so 17, which is the mr of ammonia. and this would give us 2.5. So we know that there are 2 1/2 moles of ammonia in the 42.5 g. We can also rearrange the formula to find the mass if we were given the number of moles. For example, what's the mass of 3 moles of carbon dioxide? Well, this time we'd multiply our 3 moles by the MR of CO2, which is 12 + 2 * 16, so 44, which gives us 132 g of CO2. We can also work out the mass of a particular element within a larger compound like the mass of carbon in 3 moles of carbon dioxide. For this, all we do is take the number of moles, which is 3, and multiply it by the m of the carbon, which is 12. So 3 * 12, which gives us 36 g of carbon in our 132 g of carbon dioxide. And if you wanted to go one step further, you could subtract that 36 g from the original 132 g to find that there must be 96 g of oxygen. because we can see that CO2 is only made up of carbon and oxygen. Now the last thing we need to mention is that when you look at a chemical equation you can think of them in terms of moles. So for this equation we can think of one mole of magnesium reacting with two moles of hydrochloric acid to form one mole of magnesium chloride and one mole of hydrogen gas. And you should think of these as ratios. So, if we started with two moles of magnesium, we'd have to react it with four moles of hydrochloric acid and it would produce 2 mo of magnesium chloride and hydrogen gas. If you haven't heard yet, you can find all of our videos on our website, cognito.org. You'll also find questions, flashcards, exam style questions, and pass papers. And we track all of your progress so that you always know what to study next. So sign up for free by clicking here or browse our playlist here on YouTube.