[Music] hi and welcome back to free science lessons. co.uk by the end of this video you should be able to use the idea of moles to calculate the masses of reactants and products and chemical reactions this is higher tier only this is a big topic and it can seem tricky so I've splitted over several videos by now you should be getting comfortable with the idea of moles and ch chry we're going to be using that idea a lot in this video so let's get started first of all remember that we calculate the number of moles using this equation the number of moles is the mass of a chemical divided by the relative atomic mass we use this equation when we're looking at elements however if we're looking at compounds then we use this equation which is almost the same the number of moles is the mass divided by the relative formula mass now if you cannot calculate relative fora mass then you should go back and watch my video on that topic before going any further with this video now we can use the idea of moles to calculate the masses of chemicals that take part in chemical reactions I'm going to show you how to do this it's much easier than it looks here's a typical question calculate the mass of magnesium chloride that could be produced from 72 G of magnesium assume that the chlorine is unlimited so how do we tackle a question like this first we need to look at the chemical equation on the left hand side we've got magnesium and chlorine and on the right hand side we've got magnesium chloride you can see that we've got no large numbers in front of these chemicals now in chemistry if there's no large number then that means one so in this case I'm going to show the numbers even though we don't normally write the number one that's going to help you see what's going on so what do these numbers actually mean well they tell us that if we had one mole of magnesium we could make one Mo of magnesium chloride now you'll notice that I'm ignoring the chlorine and that's because the question tells us that it's unlimited so we don't need to consider it okay so the question tells us that we've got 72 G of magnesium the first thing we need to do is calculate the number of moles of magnesium that we have so the number of moles is the mass divided by the relative atomic mass 72 ID 24 means that we've got three moles of magnesium in our reaction now we know from the chemical equation that one mole of magnesium will produce one mole of magnesium chloride therefore three moles of magnesium will produce 3 moles of magnesium chloride we need to calculate the mass of magnesium chloride produced so to do that we go back to our equation the number of moles equals the mass divided by the relative formula mass we can rearrange this equation to work out the mass the mass in GRS equals the number of moles multiplied by the relative formula Mass the relative formula mass for magnesium chloride is 95 so 3 * 95 gives us a mass of 285 G and that's our final answer here's one for you to try calculate the mass of calcium sulfate that could be produced from 80 g of calcium assume that the sulfuric acid is unlimited pause the video and try this yourself we start by calculating the number of moles of calcium from the mass that we've been given which is 80 g so the number of moles is the mass divided by the relative atomic mass the relative atomic mass of calcium is 40 so we've got two moles of calcium there are no big numbers in the equation so that means that one mole of calcium could produce one mole of calcium sulfate we've got two moles of calcium so that means that we could produce 2 moles of calcium sulfate we can calculate the mass of this by multiply the number of moles by the relative formula mass the relative formula mass of calcium sulfate is 136 so this gives us a mass of 272 g and that's our final answer here's another one for you calculate the mass of calcium carbonate that we would need to produce 224 G of calcium oxide pause the video and try this yourself so we need to make 2 24 G of calcium oxide we find the number of moles by dividing the mass by the relative formula mass the relative formula mass of calcium oxide is 56 224 ID 56 means that we need four moles of calcium oxide there are no big numbers in the equation so this means that one mole of calcium carbonate could produce one mole of calcium oxide this means that we need four moles of calcium carbonate to calculate the mass we multiply the number of moles by the relative formula mass of calcium carbonate which is 100 this means that we need 400 G of calcium carbonate and that's our final answer you'll find plenty more questions on calculating reacting masses in my vision workbook which you can get by clicking on the link above in the next video we'll continue looking at how to calculate reacting masses looking at more complicated examples [Music]