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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