In this video I’m going to go over how

to calculate percent composition so here’s our formula we’re going to need a

few different things and let’s go ahead and do this example so let’s find the

percent composition of all elements in h2o so we can see by looking at the

denominator of our formula here that we’re going to need the molecular weight

and so that’s our first step is just to find the molecular weight and so to find

the molecular weight we’re going to need the atomic weight and how many of each

element we have in that molecule and so the atomic weight we find off the

periodic table and so here we go I have the squares from hydrogen and oxygen and

let’s go ahead and fill this out all right so what I did was I took the

atomic weights from both of these periodic table squares I plugged them in

here in here and then I just followed the subscripts to get how many of each

element I had so for water I had a subscript of 2 here for H so I put in 2

right here and for oxygen there was no subscript which means there’s an implied

1 so I put a 1 in right here all right and so after you add those together I

got a total of 18 point O 1 5 a.m. you for my molecular weight

amu stands for atomic mass unit but you can also do grams per moles for these

and eight I just happened to round to five sig figs it depends on what you’re

doing for your problem to know how many sig figs you want to keep so now we have

the molecular weight so we can go ahead and plug everything in into our percent

composition formula and so I’ve outlined it here this is basically everything

we’re going to need to plug in so let’s go ahead and just look up at our box up

on top and so first we need the atomic weight of each element so we can still

take that from the periodic table then we’re going to need how many of each

element do we have in the molecule so this is still back to the subscripts so

for H we have a subscript of 2 for oxygen we have no visible subscript that

means we have 1 and now our denominator is the molecular weight that we got over

here from step one so we can plug that into both alright so now we just need to

crunch these numbers and go ahead and do the multiplication and so when we do

that for H we get 11 point one eight nine percent and when we plug in for

oxygen we get eighty eight point eight one one percent and so just a couple of

notes on percent composition here one good double check for these problems is

that you always want to make sure your total adds up to a hundred percent right

so at the end of the problem every single element needs to add up to a

hundred percent of the total weight so here we can see that if we added up our

two numbers eleven point one eight nine and eighty eight point eight one one

that would give us a hundred percent so we’ve done that correctly

and then another thing we always multiply by a hundred in this formula

because we are doing a percent and then I think just something that you want to

pick up with on these Mon these problems is just to notice the difference between

how many of each element you have and what’s the total mass percent you have

is because hydrogen we actually have twice as many as oxygen right we have

two compared to one but the total weight is only about 11% much much less than

just that one oxygen and that’s just of course because oxygen is a much heavier

element than hydrogen it has a lot more protons and neutrons it makes it a lot

heavier so just because there are more of a certain element doesn’t mean that

takes up more of the mass that’s kind of a big lesson from percent composition

alright I hope this breakdown was useful any percent composition of any molecule

can be found this same exact way just how many elements you have you have to

make sure you do this little formula for each one and that’s going to give you

the percent composition alright hope this was helpful and happy studying hey

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my other tutorials and as always happy studying