[Music] hi and welcome back to free scienceless.co.uk by the end of this video you should be able to state what's meant by relative formula mass you should then be able to calculate the relative formula mass for a compound we've already seen that the relative atomic mass of an element is the average mass of the isotopes of that element weighted to take into account the abundance of each isotope and remember that the word abundance tells us how common an isotope is the element chlorine has got two main isotopes both have an atomic number of 17 as all isotopes of chlorine have got 17 protons in their nucleus this isotope of chlorine has a mass number of 35 because it has 18 neutrons in its nucleus whereas this isotope has 20 neutrons in its nucleus so it has a mass number of 37 if you look on the periodic table you can see that the mass number of chlorine is actually 35.5 this is the average of 35 and 37 but it's weighted for the abundance of each isotope chlorine 35 is more abundant than chlorine 37 in this video we're looking at relative formula mass so here's the definition the relative formula mass of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula relative formula mass has got this symbol now there are a couple of key facts that you need to know about relative formula mass firstly relative formula mass has got no units secondly relative formula mass never involves big numbers let's look at an example this shows the formula of methane we can see that we've got one atom of carbon bonded to four atoms of hydrogen i'm going to calculate the relative formula mass of methane in the exam you're given the relative atomic masses that you need so here they are carbon has a relative atomic mass of 12 and hydrogen's got a relative atomic mass of one so because we've got one carbon atom we've got 1 * 12 because we have four hydrogen atoms then we have 4 * 1 adding all of these together gives us a relative formula mass for methane of 16 and remember that there are no units for relative formula mass take a look at this formula i've now put a large two in front of the formula for methane this means that we now have two molecules of methane now it's important to remember that we do not use big numbers to calculate relative formula mass so the relative formula mass is still 16 this is the formula for calcium sulfate i'd like you to pause the video now and work out the relative formula mass for this compound okay we've got one atom of calcium and calcium's got a relative atomic mass of 40 we've got one atom of sulfur and sulfur's got a relative atomic mass of 32 finally we've got four atoms of oxygen and oxygen's got a relative atomic mass of 16 adding all of these together gives us a relative formula mass of 136 here's one more for you to try this is magnesium hydroxide again pause the video and work out the relative formula mass for this compound we've got one atom of magnesium and magnesium has a relative atomic mass of 24 we've got two atoms of oxygen and oxygen's got 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 magnesium hydroxide of 58 and again remember that relative formula mass has got no units you'll find plenty of questions on calculating relative formula mass in my vision workbook which you can get by clicking on the link above okay so hopefully now you should be able to state what's meant by relative formula mass you should then be able to calculate the relative formula mass for a compound [Music]