[Music] hi and welcome back to free science lessons. co.uk by the end of this two-part video you should be able to use avagadro's constant to calculate the number of molecules and atoms in a given sample this work applies only to higher students so if you're doing Foundation you don't need to do this now I'm going to be honest with you some of this is pretty challenging please trust me that you can do this and don't give up you might need to watch the video a couple of times we've been looking at moles and we've already seen that one mole is simply a shorthand way of saying this number scientists call this number avagadro's constant normally we write this in standard form which is 6.02 * 10 ^ 23 before we start working with avagadro's constant we're going to look at how we can calculate the number of moles of atoms in a given number of moles of a molecule here's a sample question calculate the number of moles of atoms in one mole of water molecules and water has a formula H2O looking at the formula we can see that one molecule of water contains two atoms of hydrogen and one atom of oxygen so that makes three atoms in total so if one molecule of water contains three atoms then one mole of water molecules must contain three moles of atoms here's one for you to try calculate the number of moles of atoms in one mole of methane molecules pause the video and try this yourself we can see that one molecule of methane contains one atom of carbon and four atoms of hydrogen in other words five atoms in total because one molecule of methane contains five atoms we know that one mole of methane molecules must contain five moles of atoms try this one calculate the number of moles of atoms in one mole of calcium hydroxide pause the video now and work this out okay we can see that one molecule of calcium hydroxide contains one atom of calcium two atoms of oxygen and two atoms of hydrogen so five atoms in total so we know that if one molecule of calcium hydroxide contains five atoms then one mole of calcium hydroxide must contain 5 moles of atoms okay so now that we can calculate the number of moles of atoms we're ready to start working with avagadro's constant take a look at this question calculate the number of atoms in one mole of hydrogen chloride you'll notice that now we're asked for the number of atoms not the number of moles of atoms remember that one mole of hydrogen chloride contains avagadro's constant of molecules in other words 6.02 * 10 ^ 23 the formula of hydrogen Chlor is HCl this means that one molecule of hydrogen chloride contains one atom of hydrogen and one atom of chlorine so two atoms in total so what this means is that if we have 6.02 * 10 ^ 23 molecules of hydrogen chloride we must have two times that number of atoms so in other words we must have 1.24 * 10 ^ of 24 atoms here's one for you to try calculate the number of atoms in one Mo of sodium oxide na2o pause the video and try this yourself okay 1 mole of sodium oxide contains 6.02 * 10 ^ 23 molecules of sodium oxide in other words avagadro's constant looking at the formula of sodium oxide we can see that one molecule contains two atoms of sodium and one atom of oxygen in other words three atoms in total therefore we need to multiply the number of molecules of sodium oxide by three to get the number of atoms in one mole of sodium oxide this gives us a final answer of 1.86 * 10 ^ 24 atoms you'll find plenty more questions on using avagadro's constant in my revision workbook which you can get by clicking on the link above in the next video we'll continue exploring how to use avagadro's constant to answer more compx licated questions [Music]