[Music] hi and welcome back to free science lessons. 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 in the last video we started looking at calculating reacting masses and we saw equations such as this one we saw that this tells us that one mole of magnesium plus 1 mole of chlorine makes 1 mole of magnesium chloride in this video we're going to look at equations such as this one here you can see that we've got a large two in front of the hydrochloric acid so how do we deal with numbers such as these let's look at a sample question calculate the mass of magnesium chloride that could be produced from 146 G of hydrochloric acid the first thing we need to do is look at the big numbers in the equation we've got a large two in front of the hydrochloric acid and we've got no number in front of the magnesium chloride this tells us that two moles of hydrochloric acid make one Mo of magnesium chloride so we know that we've got 146 G of hydrochloric acid we need to calculate the number of moles the number of moles is the mass divided by the relative formula mass we have 146 G and the relative formula mass for hydrochloric acid is 36.5 now remember that large numbers are never used to to calculate relative formula mass putting these into the equation tells us that we have four moles of hydrochloric acid the chemical equation tells us that two moles of hydrochloric acid make one mole of magnesium chloride so we have to divide our number of moles by two so four moles of hydrochloric acid will make 2 moles of magnesium chloride to calculate the mass we need to multiply the number of moles which is two by the relative formula mass of magnesium chloride which is 95 this gives us a mass of 190 G and that's our final answer so as you can see tackling these kinds of questions is relatively straightforward here's one for you to try calculate the mass of sodium sulfate that could be produced from 240 G of sodium hydroxide pause the video now and try this one yourself okay first we need to look at the big numbers in the equation we can see that two moles of sodium hydroxide makes one mole of sodium sulfate we now need to calculate the number of moles of sodium hydroxide that we have the number of moles is the mass divided by the relative formula mass the relative formula mass of sodium hydroxide is 40 240 ided 40 tells us that we've got 6 moles of sodium hydroxide as we've seen 2 moles of sodium hydroxide make one Mo of sodium sulfate so we have to divide our number of Mol M by two so 6 moles of sodium hydroxide will make 3 mol of sodium sulfate to calculate the mass we multiply the number of moles by the realtive formula mass the realtive formula mass of sodium sulfate is 142 so multiplying 3 moles by 142 gives us 426 G and that's our final answer here's one more question for you to try calculate the mass of hydrogen peroxide that could produce 64 G of oxygen again pause the video and try this for yourself okay let's start by looking at the chemical equation we can see that two moles of hydrogen peroxide are needed to make one mole of oxygen we can calculate the number of moles of oxygen that we need using the equation number of moles equals the mass divide by the relative formula mass the mass is 64 G and the relative formula mass of oxygen is 32 this means that we're making 2 moles of oxygen we know that we need 2 moles of hydrogen peroxide to make one mole of oxygen so that means that we need four moles of hydrogen peroxide to make two moles of oxygen to calculate the mass of hydrogen peroxide we need to multiply the number of moles which is four by the relative formula mass which is 34 this gives us a mass of 136 G and that's our final answer remember that you'll find plenty more questions on calculating reacting masses in my vision workbook which you can get by clicking on the link above okay so hopefully now you should be able to use the idea of moles to calculate the masses of reactants and products and chemical [Music] reactions