we're told that a 0.450 gram potassium supplement contains 22 percent potassium by mass the potassium is present in the supplement as potassium chloride which has a molar mass of 74.55 grams per mole how many grams of potassium chloride are in the potassium supplement so pause this video and see if you can figure that on your own and i will give you a hint you might need a periodic table so i'll give you one right over there all right now let's work through this together so they already give us the molar mass of potassium chloride but it'll be useful for us to know the molar mass of potassium itself and we can see it's 39.10 so for potassium it is 39.10 that's its average atomic mass but you could also view it as 39.10 grams per mole of potassium i could write it like that and that is i think all we will need the periodic table of elements for and so how do we think about how many grams of potassium chloride are in the potassium supplement well what i could do is i could say all right what's the total mass of my supplement and multiply it by 22 percent to figure out what is the mass of potassium i have so let me figure that out so mass of potassium that is going to be my 0.450 grams times 22 percent which is going to be equal to i'll get my calculator out 0.450 times 22 or 0.22 is equal to 0.099 0.099 so that's how many grams of potassium i have and now i can use that to figure out how many moles of potassium i have and if i know how many moles of potassium i had all the potassium is present as potassium chloride because then i'll have the same number of moles of potassium chloride and then i could figure out well based on the and if i know the number of moles of potassium chloride well then i know the mass of potassium chloride because they give us the molar mass so let's do that so we have 0.099 grams of potassium and i want to figure out how many moles of potassium i have so i'm going to multiply that times one mole of potassium has a mass of how many grams of potassium and we can see it right over there it's 39.10 grams per mole so 39.10 one mole is equivalent to 39.10 grams if we're dealing with potassium so this calculation will tell me moles of potassium now i have exactly one potassium atom for every potassium chloride molecule and so whatever this number is i'm going to have the same number of moles of potassium chloride but just to make that clear i will write this down i have one mole of potassium chloride for every one mole of potassium and you can already see how the units will cancel out that will cancel with that that will cancel with that and so this calculation is going to tell me moles of potassium chloride but i want to know the mass of potassium chloride so then i will multiply this by the molar mass times 74. and i'm going to do this another color just makes it look nice 74.55 grams of potassium chloride for every mole of potassium chloride and so then this will cancel with this and this will tell me how many grams of potassium chloride i actually have so let's get the calculator out again so i have 0.099 divided by 39.10 divided by 39.10 times 1 times 74.55 times 74.55 is going to be equal to that and then let me think about my significant figures or my significant digits here so see out of all of the information i used i had this i have three significant figures here i have three here i have four here i have four here so i'm multiplying a bunch of things and dividing so i have to take the minimum number of significant figures which is three so i will round to three significant figures which is zero point one eight nine grams so this is i'll do this in a new color zero 0.189 grams of potassium chloride which is exactly what they are asking us and we are done