[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 remember that this work applies only to higher students so if you're doing Foundation then you don't need to do this in the last video we saw that one mole is also called avagadro's constant and it has a value of 6.02 * 10 ^ 23 we learned how to calculate the number of atoms in a given number of moles of molecules in this video we're going to take this one stage further we're going to learn how to calculate the number of atoms in a given mass of a chemical here's a sample question calculate the number of atoms and 48 grams of magnesium as you can see we've not been given the number of moles we need to calculate this from the mass and we use this equation which we've seen in the previous video the number of moles equals the mass we're given in grams divided by the relative atomic mass so we've got 48 G of magnesium and the relative atomic mass of magnesium is 24 remember that you are given all the numbers that you need in your exam so this means that we've got two moles of magnesium remember that one mole is avagadro's constant which is 6.02 * 10 ^ of 23 therefore in 2 moles of magnesium the total number of atoms is avagadro's constant multiplied by two this gives us a final answer of 1.24 * 10 ^ of 24 atoms here's one for you calculate the number of atoms and 28 G of lithium and the relative atomic mass of lithium is seven pause the video and try this for yourself okay so first we need to calculate the number of moles of lithium that we have 28 / 7 tells us that we've got four moles remember that one mole contains avagadro's constant of atoms so to calculate the number of atoms we multiply 6.02 * 10 ^ of 23 by 4 which gives us a final answer of 2.48 * 10 ^ of 24 atoms now you could be asked to work out the number of atoms in a compound and that's also fairly straightforward here's a sample question calculate the number of atoms in 56 G of calcium oxide this has the formula CAO now calcium oxide is a compound so to calculate the number of moles we need to use this equation the number of moles is the mass in grams divided by the relative formula mass first we need to work out the relative formula mass of calcium oxide calcium oxide has one atom of calcium and one atom of oxygen calcium has a relative atomic mass of 40 and oxygen has a relative atomic mass of 16 so the relative formula mass will be 40 + 16 giving us a value of 56 so we can now calculate the number of moles by dividing 56 by 56 giving us one Mo of calcium oxide we know that the number of molecules in one mole is avagadro's constant in other words 6.02 * 10 ^ 23 so we've got 6.02 * 10^ 23 molecules of calcium oxide but remember that the question's asking us to calculate the number of atoms not the number of molecules each molecule of calcium oxide contains two atoms one atom of calcium and one atom of oxygen so we need to multiply the number of molecules by two to get the total number of atoms so the total number of atoms in 56 gr of calcium oxide is 1.24 * 10 ^ 24 and that's our final answer here's one for you to try calculate the number of atoms and 54 grams of water and water has the formula H2O pause the video and try this yourself okay so first we've got to calculate the number of moles of water molecules and to do that we need to know the relative formula mass for water water contains two atoms of hydrogen and hydrogen has a relative atomic mass of one water also contains one atom of oxygen and oxygen has a relative atomic mass of 16 adding these numbers together gives us a relative formula mass for water of 18 to calculate the number of moles we divide the mass that we're given by the relative formula mass so we've got 54 divided by 18 which means that we've got three moles of water we know that the number of molecules in one mole is 6.02 * 10 ^ of 23 in other words avagadro is constant however we've got 3 moles of water so that means that we've got 1.86 * 10 ^ 24 molecules of water now the question's asking for the total number of atoms and remember that each molecule of water contains three atoms this means that the final number of atoms will be 5418 * 10 ^ 24 you'll find plenty of questions on using avagadro's constant in my vision workbook which you can get by clicking on the link above okay so hopefully now you should be able to use avagadro's constant to calculate the number of molecules and atoms in a given sample [Music]