Find the Empirical Formula Given Percents

If you're given the Percent Composition of a compound, you can find the Empirical Formula for it. I have the shortest method ever to do it, although it's not a "full solution" like your teacher probably asks for.

1. Divide each % by the atomic mass of the element
2. Divide each of THOSE answers by whatever's smallest
3. Adjust these numbers into their lowest whole-number ratio.

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Closed Caption:

what's up players the size matter
the answer is No as long as you're using
it properly
which is why I have this small periodic
table i'm going to show you how to use
this periodic table and percent
composition data to find the empirical
formula of a compound for example if you
find some unknown white powder on the
train or something you want to know what
it is
you could take it to your lab you can
run through a machine and the machine
will tell you that's forty-eight percent
carbon eight percent hydrogen and
forty-three percent oxygen
so what's the chemical well we can get
clues based on this percent composition
the empirical formula that we are asked
for here is simply the formula with the
lowest whole number subscripts and what
I mean by that is suppose that this
compound ended up being c6h6 the
empirical formula for this is simply see
H because you keep the ratio of 626
but you reduce it down to the lowest
possible whole numbers one to one
another example might be c 4 h 6o -
they're all divisible by 2 so the
empirical formula for that would be c 2h
30
notice how we all / - it's still the
same ratio of atoms but we have the
lowest possible whole numbers
that's what we're finding here and boy
do I have a way for you to do that
all you need to do is divide each
percent that you're given by the atomic
mass of the element divide each of those
values by which everyone smallest and
then adjust those so that there are low
whole number ratio
all right step 1
divide each percent by its atomic mass
let's do this
carbon 48.3 eight percent / its atomic
mass 12.1 you know how I knew cuz i
looked it up on the table when you do
that on your calculator you end up with
48.3 8/12 . 01 gives me four point oh
three four point oh three
we gotta do this for the others here is
my hydrogen 8.12 % / the attack mass in
the periodic table 8.1 2 divided by 1
point 0 1 gives me 8 . 04 and my oxygen
oxygen is going to end up being 43.5 /
16 point out
% / talk mass 43.5 / 16 gives me 2.72
all right
pretty sweet divide each of those
answers by whichever smallest for a to
this one smallest so i'm going to do is
divide every single one of these x 2.7 -
here we go for . 03 / 2.72 I got 1.5 of
these eight . 04 / 2.72 gives me 2.96
actually it's pretty close to three and
2.7 t / 2.72 is clearly one
so my ratio here seems to be 1.5 to 3 2
1
there's always going to be a little
rounding things you're gonna have to
watch out for be okay with rounding
around too much but if it's a point nine
rounded to the nearest whole number and
the last step is to find the lowest
whole number ratio but this is still a
decimal not a whole number
how can we get rid of a point five or a
half the answer is by multiplying by 2
if we multiply this by two we get three
if we multiply this by two we get six if
we multiply this by 2 you get to what
this means is that we have three carbons
for every six hydrogen's for every two
oxygens now what you'll notice is that
that's the same ratio as for 28 - 2.7
but we've adjusted these of their whole
numbers
long story short the empirical formula
for this is c 3 c 3 h 6
oh 2 3 and six and two don't have any
common factors other than one and so we
can't reduce that similar to how we
reduce fractions
that's the empirical formula we want or
even a circle it to celebrate
it's as easy as this divide each of the
percents by the atomic mass divided each
of those by whatever smallest that's
just my little trick for making the
numbers easier to work with and then
round / x whatever you need to get whole
number ratios
best of luck