Robin Reaction dot com. in this video I'm
going to go over how to find the
empirical formula from mass also called
combustion analysis and so our question
reads a thirty three point oh eight gram
sample of an organic compound was
combusted in oxygen and produced eighty
point seventy three gram carbon dioxide
and thirty three point zero five gram
water what is the empirical formula of
the compound all right so we're going to
find the answer to this by doing a
series of steps so step one is to find
the mass of the carbon in the co2 and
the mass of the hydrogen in the h2o
using percent composition and so let's
go ahead and break down this word
problem into what we're actually trying
to do so when it says we have 30 3.08
grams of an organic compound what that
means is that it's going to contain
carbon hydrogen and possibly oxygen
and I've put XY and Z here because we
don't know how many carbons hydrogen's
and oxygens we have in fact that's the
whole purpose of this entire problem is
to figure that out so those are our
mystery numbers for right now and then
it says combusted in oxygen so that
means that it's going to be reacted with
the o2 in the atmosphere and then we
were given the two products that were
created after this reaction which was
carbon dioxide and water all right so
let's go ahead and fill in all the
masses that we know so our mystery
sample is 33.8 our carbon dioxide is 80
point 73 and our water is 33 point oh
five so the next step is that we're
going to find the mass of the carbon in
the co2 and the mass of the hydrogen and
the h2o using percent composition so
conceptually what you really want to
understand for this is that we had this
mystery compound we basically set it on
fire we exploded it we collected carbon
dioxide and water and so the key thing
here is that any carbon that's in the
carbon dioxide came from our mystery
compound carbon can't just make itself
up it didn't come from the atmosphere it
didn't come from the o2 so when we say
that we have eighty point seventy three
grams of carbon dioxide a hundred
percent of the carbon that was in the
mystery compound is now in the carbon
dioxide and that's the same thing for
h2o we didn't get hydrogen from the
atmosphere or from the o2 the only place
we can get hydrogen from is from that
mystery compound so if we figure out how
much H we have in the h2o will know how
much H was in the mystery compound
that's the whole point of this problem
is we're going to kind of back solve
we're gonna say we were given our final
answer let's figure out how much we
started with right now
we don't know if our empirical formula
of our compound has oxygen in it we
weren't told and we're gonna have to
figure it out by figuring out how much
carbon we have how much hydrogen we have
and then figuring out does that add up
to thirty 3.08
so let's go ahead and
about how much carbon and how much
hydrogen we have by weight we're going
to do that with percent composition so
first we need the molar mass of our two
known compounds so the molar mass of
carbon dioxide is forty three point nine
nine grams per mole and the molar mass
of water is eighteen point zero one
grams per mole so now doing percent
composition I'm going to take the mass
of carbon and we only have one for co2
so that's going to be twelve point zero
one divided by the molar mass of carbon
dioxide and so we get the amount of C
that we had in co2 was twenty seven
point three zero percent and so now
doing the same thing so percent
composition for hydrogen is going to be
two point zero one six because we have
two hydrogen's each one weighs 1.008
divided by the molar mass of water
eighteen point O one so the H that we
have in h2o becomes eleven point one
nine percent all right so now that we
have the percent composition of any
carbon sample in carbon dioxide let's
figure out exactly how much carbon we
have in our co2 sample so we know that
we were told that this sample created
eighty point seventy three grams of co2
so what we need to do is figure out
what's twenty seven point three percent
of eighty point seventy three and so we
do that by just multiplying by the
percent
so I do my percents by just multiplying
the percent as a fraction with a
denominator of a hundred and so
multiplying 27.3% by eighty point
seventy three grams we got that there
was a total of 22 0.04 grams carbon in
our carbon dioxide sample and so that
means there's exactly 22 point oh four
grams carbon in our mystery sample as
well because remember all the carbon
that we got from co2 came from our
mystery sample so now let's go ahead and
do the same thing for hydrogen so we
were told that this water sample was
thirty three point zero five grams so we
go ahead and take that multiply it by
our percent which I like to do as a
fraction with denominator of a hundred
and so we get that we created three
point six nine grams of hydrogen in this
water which means that our mystery
sample had three point six nine grams
hydrogen so remember earlier I said we
weren't really sure if our mystery
sample had oxygen in there or not
because we had to actually use oxygen in
our reaction so we don't know if the
oxygen in co2 and h2o came from either
the o2 in the air or did it come from
the O in our mystery molecule and so for
step two we're going to go ahead and
find the original mass and subtract
carbon and H to see what is the mass of
oxygen in this molecule so remember
thirty three point oh eight that's our
mystery sample overall subtracting the
amount of carbon that we know was in the
mystery sample and then subtracting the
amount of hydrogen that we know was in
the mystery sample we get that there are
seven point three five grams left so
this means that this must be oxygen
because we already know that we have
carbon and hydrogen the only other thing
we could have is oxygen and so the
missing seven point three five grams
from our original 33 point oh eight
grams sample must belong to oxygen
alright so step three divide the mass of
each known molecule
it's molar mass to get the number of
moles in each element so once again
we're going to use how many grams we
have and now we're going to start
figuring out well how many moles does
that add up to alright so our three
known molecules are one co2 two h2o and
now we also just have the extra oxygen
that we found in step two so let's go
ahead and we're going to divide the mass
of our known molecule by its molar mass
and that will give us the number of
moles of each element alright so for
these we're using eighty point seventy
three because that's the number of grams
of carbon dioxide we made we're using
thirty three point zero five grams for
water because that's one of water that
we made and now for oxygen we're just
going to use the molar mass of a single
oxygen atom so we know from step number
two we created seven point three five
grams of oxygen
all right so these are the numbers we
get and then one more little addition we
have two hydrogen's in h2o so to get the
number of moles we're going to have to
multiply our hydrogen number by two
all right so now we have how many moles
of each of our elements right here
alright so step four divide each number
of moles by the lowest amount of moles
this is your mole ratio and your
empirical formula is found directly
through ratios because remember the
empirical formula only tells me the
ratio of the different elements involved
in the molecule it does not tell me the
number of different elements involved in
the molecule that's what the molecular
formula does so we're just looking for a
ratio sometimes the empirical and
molecular formula are the same sometimes
they're not alright so what we mean by
this rule is the lowest number so that
is this guy we're going to take our
other numbers and divide by that and
that's because we know in a molecule
every single molecule is going to have
at least one of each atom that it has
right and we know since there was extra
mass that this molecule definitely has
oxygen and so we know that there's going
to be at least one oxygen atom and so
that's kind of our ratio for the number
one so let's go ahead and divide
alright let's calculate all of our mole
numbers
and so you're often gonna get kind of
scraggly numbers like this and you can
go ahead and just round them to their
nearest whole number so three point nine
nine two is pretty much four and seven
point nine eight five is pretty much
eight and then one is going to be one so
these are our answers we just figured
out by dividing by the number of moles
that our empirical formula has four
carbons eight hydrogen's and one oxygen
alright so a couple things here as we
wrap up one remember this is not the
molecular formula this is just the ratio
so possibly our molecular formula could
be c8 h 1602 that's totally fine but
this is the lowest whole number ratio of
the elements in this mystery compound
and then second if this was confusing
that's totally normal
this is a very complicated problem the
main thing to kind of keep in mind as
you study more and repeat until you
understand these types of problems is
that way back to step number one it's
all about knowing that okay yes we made
80 point 73 grams carbon dioxide what
that really means is that all the carbon
in that carbon dioxide came from our
mystery molecule so we can figure out
how much carbon came from that carbon
dioxide pile we know exactly how much
was in our mystery molecule and the same
thing goes for hydrogen and then all we
did after that was we said okay we know
how much was our total we know how much
carbon and we know how much hydrogen so
unless carbon hydrogen added up to the
complete mass we know whatever leftover
mass has to be oxygen once we have our
three masses we can go ahead and find
our number of moles of each different
element and then we use our number of
moles to get a mole ratio all right good
luck with these problems and happy
studying hey I hope you liked that video
please feel free to like comment or
subscribe
go to my website I have a ton of free
practice problems you can check out if
you need even more help you can hire me
for one-on-one private tutoring sessions
that are online
alright thanks that's it happy studying
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