[Music] hi and welcome back to freci lessons. co.uk by the end of this video you should be able to calculate the mass needed for a given number of moles of a compound in the last video we saw that we can calculate the number of moles of a compound in a given mass by using this equation the number of moles equals the mass in grams divided by the relative formula mass and this video we're going to see how to calculate the mass needed for a given number of moles here's a sample question calculate the mass of four moles of sodium chloride and sodium chloride has the formula NAC in order to calculate this we need to rearrange the equation to calculate mass and here it is the mass in grams equals the number of moles multiplied by the relative formula mass so let's look at how we answer the question first we need to calculate the relative formul mass of sodium chloride we've got one atom of sodium and the relative atomic mass of sodium is 23 we've also got one atom of chlorine and the relative atomic mass of chlorine is 35.5 adding these together gives us a relative formula mass of 58.5 for sodium chloride so now we can go back to our equation and answer the question the mass in grams is the number of moles multiplied by the relative formula mass so we've got four moles multiplied by 58.5 this gives us a total mass of sodium chloride of 234 gram here's a question for you to try calculate the mass of three moles of potassium oxide the formula of pottassium oxide is k2o and I've given you the relative atomic masses that you need you should pause the video now and try this yourself okay first we need to work out the relative formula mass for potassium oxide we've got two atoms of potassium and pottassium has a relative atomic mass of 39 we've got one atom of oxygen and oxygen has a relative atomic mass of 16 adding these together gives us a relative formula mass of 94 so now we can go back to our equation we've got to calculate the mass of 3 moles of potassium oxide so we multiply the number of moles by the relative formula mass and this gives us a final mass of 282 G here's another question for you to try calculate the mass of 0.1 moles of cesium 9 nitrate cium nitrate has the formula CS3 I've given you the relative atomic masses that you need so pause the video now and try this yourself okay so first we'll work out the relative formula mass of cesium nitrate we've got one cesium atom and cesium has a relative atomic mass of 133 we've got one nitrogen atom and nitrogen has a relative atomic mass of 14 finally we've got three oxygen atoms and oxygen has a relative atomic mass of six 16 adding all of these together gives us a relative formula mass for cium nitrate of 195 going back to our equation we can now calculate the mass of .1 moles of cium nitrate so the mass is the number of moles multiplied by the relative formula mass multiplying .1 by 195 gives us a mass of 19.5 G and that's our final answer okay here's one final question for you to try calculate the mass 5 moles of copper sulfate the formula of copper sulfate is cuso4 and I've given you all of the relative atomic masses that you need you should pause the video now and work this out for yourself okay we'll start by working out the relative formula mass for copper sulfate we've got one atom of copper and copper has a relative atomic mass of 63.5 we've got one sulfur atom and sulfur has a relative atomic mass of 32 and finally we've got four oxygen atoms and oxygen has relative atomic mass of 16 adding these together gives us a relative formula mass for copper sulfate of 1595 going back to the equation to calculate the mass we multiply the number of moles by the relative formula mass this gives us a final mass of copper sulfate of 7975 G you'll find plenty more questions on calculating the mass of a given number of moles 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 mass needed for a given number of moles of a compound [Music]