many times
when we're talking about
yields of reactions we talk about theoretical yield
and theoretical yield is important because theoretical yield will tell you
what quantity of material you should get
if everything is perfect we all know that in most cases things
are not perfect and sometimes a better gauge
on a reactions yield is percent yield
and percent yield is the actual yield
that you obtain from your reaction divided by its theoretical yield
multiplied by 100 percent yield then tells you
how efficient you are in processing the reaction
in industry percent yield is very important because
low yields tend to cost money
they tend to waste materials and they certainly
are something that management and operators are not
happy about so let's talk about how we calculate a percent yield
from a reaction if we look at this reaction
where we began with 2 moles of hydrogen
and 2 moles of oxygen and our yield in this reaction
was 24 grams that's what we actually got out of it
what we would do in order to determine our percent yield
would be to determine our theoretical yield
divided into 24
and multiply by 100 in order to determine theoretical yield
we have two know what the limiting reagent is and by calculations we
can determine
that hydrogen would be our limiting reagent
here since there's two moles of hydrogen lets decide
how much water we should
form in our process
2 moles of hydrogen times
2 moles of
water diveded by
2 moles of hydrogen
is our ratio water
the hydrogen note that moles of hydrogen will cancel
note that your twos can cancel
and note that if we start with two moles
of hydrogen we will get
two moles of water theoretically
now teo moles of water
how do we compare two moles of water with .24 grams of water
that's a question if you remember you can use molar mass
in order to convert moles to grams
for water the molar mass of water
is equal to 18.02
grams we can set this
up as a ratio and we can say
that there are 18.02
grams of water in one mole
in one mole now
we learned that the theoretical yield for this process
is two moles so if we multiply this fraction times two moles
of water our moles will cancel
and we're left with
36.04 grams of water
once we have grams here
it is an easier it's an easier comparison between grams and grams
so in order to get our percent yield we're going to divide 24 grams
by 36.04
grams we're gonna multiply that
times a hundred
and what we're gonna find out is that
our percent yield was close to
75 percent now
in some processes 75 percent would be considered
a good yield while in other processes
it would be considered a bad yield
it depends on the complexity of the process
and other factors as to whether that
it would be the case
percent yields and theoretical yields
are in some ways a capstone for your stoichiometry
because they allow you to use many different facets of stiochiometry
to get this information I hope that you'll practice it
I hope that you'll master it and that
you'll know how to do this in the future