[Music] hi and welcome back to frees science lessons. co.uk by the end of this video you should be able to calculate the number of moles of a compound this material is for higher tier only in the last video we saw that a mole is simply a very large number which I'm showing you here we'd be looking more closely at this number in a later video we saw that if we're given a mass of an element we can use this to calculate the number of moles of the element we use this equation the number of moles is the mass and grams divided by the relative atomic mass for that element now in this video we're going to learn how to calculate the number of moles of a compound in order to do this we need to revise the idea of relative formula mass which we saw in a previous video this shows the compound carbon dioxide we can see that we've got one atom of carbon bonded to two atoms of oxygen we know that carbon atoms have a relative atomic mass of 12 and oxygen atoms have a relative atomic mass of 16 adding these numbers together gives us the relative formula mass of carbon dioxide and we can see that this is 44 the symbol for relative formula mass is M and remember that relative formula mass has got no unit so in this video we're looking at how to calculate the number of moles of a compound to do this we use this equation the number of moles equals the mass that we're given in GRS divided by the relative formula mass Mr as you can see this looks very similar to the last equation except that now we're dividing by relative formula mass and not relative atomic mass let's look at an example you're given a sample of calcium carbonate with a mass of 300 G calculate the number of moles of calcium carbonate in the sample in order to calculate this first we need to work out the relative formula mass for calcium carbonate calcium carbonate has the formula CA Co three we've got one atom of calcium and calcium has a relative atomic mass of 40 we've got one atom of carbon and carbon has a relative atomic mass of 12 finally we've got three atoms of oxygen and oxygen has a relative atomic mass of 16 adding all of these together gives us a relative formula mass for calcium carbonate of 100 to calculate the number of moles of calcium carbonate in the sample we need to divide the mass that we're given by the real of formula mass so we've been given 300 G of calcium carbonate and the relative formula mass is 100 this means that we have 3 moles of calcium carbonate here's one for you to try you're given 380 gam of magnesium chloride how many moles of magnesium chloride have you been given the formula of magnesium chloride is mgcl2 the relative atomic mass of magnesium is 24 and the relative atomic mass of chlorine is 35.5 pause the video now and try this yourself okay first we need to calculate the relative formula mass for magnesium chloride so we've got one atom of magnesium and magnesium has a relative atomic mass of 24 we've got two atoms of chlorine and chlorine has a relative atomic mass of 35.5 adding all of these together gives us a relative formula mass for magnesium chloride of 95 to calculate the number of moles of magnesium chloride we need to divide the mass that we're given by the relative formula mass so we've been given 380 G of magnesium chloride and the relative formula mass is 95 this means that we've got four moles of magnesium chloride here's another one for you to try you're given a sample of lithium sulfate with a mass of 990 gam calculate the number of moles of lithium sulfate in the sample the formula of lithium sulfate is li2 so4 the relative atomic mass of lithium is seven the relative atomic mass of sulfur is 32 and the relative atomic mass of oxygen is 16 pause the video now and try this yourself okay so we start by calculating the relative formula mass of lithium sulfate we've got two atoms of lithium and lithium has a relative atomic mass of seven we have one atom of sulfur and sulfur has a relative atomic mass of 32 finally we've got four atoms of oxygen and oxygen has a relative atomic mass of 16 adding all of these together gives us a relative formula mass for lithium sulfate of 110 to calculate the number of moles of lithium sulfate we need to divide the mass that we're given by the relative formula mass so we've been given 990 G of lithium sulfate and the reative formula mass is 110 this means that we have 9 moles of lithium sulfate in our sample okay here's one final example for you to try you've been given 64.5 gram of burum hydroxide which has the formula be2 calculate the number of moles of burum Hydrox o that you've been given I've given you the Rel of atomic masses so you should pause the video now and try this for yourself okay we'll start by calculating the relative formula mass for burum hydroxide we've got one atom of berum and burum has a relative atomic mass of n we've got two atoms of oxygen and oxygen has a relative atomic mass of 16 and finally we've got two atoms of hydrogen and hydrogen has a relative atomic mass of one adding all of these together gives us a relative formula mass for burum hydroxide of 43 to calculate the number of moles of burum hydroxide we need to divide the mass that we're given by the relative formula mass so we've been given 64.5 G of burum hydroxide and the realtive formula mass is 43 this means that we have 1.5 moles of burum hydroxide remember that you'll find plenty more questions on calculating the number of moles of both elements and compounds in my vision workbook which you can get by clicking on the link above okay so hopefully now you should be able to calculate the number of moles of a compound [Music]