Male Speaker: This is how you do a question if you are asked to calculate the mole mass of a compound given the isotopes of a compound and the relative percentages that they are found in the planet, remember, that not all the carbon in the planet is found as more mass carbon 12, where in 12.1 which is what it says on the periodic table no, no, there is carbon 12 but there is also some carbon 13 as well, which has a specific mole or mass or we can say atomic mass units. Well how do I approach this question is to remember that if you are given the AMU is of both of the isotopes and you are given the percentages as well, all you have to do is turn both of these in to the masses that they make up in their percentages, they just add those masses together, boom you got yourself the answer, like here is 12 AMU that's carbon 12, and if you multiplied by 98.89 am use over all hundred, that's the percentage 98.89 % is really of course 98.89 or something over a hundred of something multiply that fraction times 12, and you'll get yourself 11.88 AMU, well remember here if you multiply that number the carbon 13, mole or masses that work multiplied by each percentage divided by hundred, you are going to get those 2 masses for the AMU and the total of those two then make up what the total mole or masses average for that compound out of a hundred percent. 11.88, 0.144 I kept 4 significant to this look like that's exactly 12, we use carbon is the standard that's exactly 12, this right here has, 4 significant digits here infinite here keep 4 I kept 1 2 3 three here is in this number this set 2 numbers after decimal 3 numbers when you add or subtract keep least number of decimal places and I kept 12.02, as pretty closer to 12.01. Just a little bit of a hint the tricky problems come from all, in the questions I will give you this and this, then I they will give here the answer here and say, with all the fact calculate the percent of each of these isotopes, and they will leave this is unknown and this number has unknown. Then how do you solve for that type of question really quick, all you have to do is, remember that you've got, this times now its going to be over a hundred but this number is going to be well it's going to be x lets say, right here, just remember that if you make this y you will be solving for two unknowns and you can do it, but systems and equations this number is also one hundred minus this number so you can put x in here and one hundred minus x here and then solve for a problem that asks you to find the percentages. Calculating mass percent of an element in a compound. Its one of the easier types of questions to be able to do, in terms of advance placement, where university level chemistry, but you know it kind of sets itself for empirical in a like in to formulas coming up, so how you calculate the mass percent of an element in a certain compound, truly quiet straight forward so here is Arsenic tribromide we want to know what's the mass of the Arsenic tribromide interms of a percent, of the whole compound. Well if you know the mole of mass of the whole compound 314.62 and you know the mass of the Arsenic and you just check on the periodic table as well, 74.92 dividing that 74.92/314.62 is going to multiplied of course by a 100 will then give you the percent is easier just don't cancel is just really a ratio of gram to the As programs and the AsBr3 and so that percentage into reflects that ratio over there 23.81% and of course there is 3 Bromines so its 3 times 79.90 divided by the total mole arise in the compound again times of hundred gives you 76.19, you add those two numbers to gather you better get yourself a 100%.