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in this video we are going to talk about
how to calculate the theoretical and
percent yield
of a compound
so the first thing we need to do is
write a balanced chemical equation
so we have c3h8 which stands for propane
and it's burning air so it's going to
react with oxygen gas
and it's going to produce carbon dioxide
and the other product is water the
reason being it's a combustion reaction
you always get co2 and water as products
so now we got to balance in whenever you
want to balance a combustion reaction
you want to balance the number of carbon
atoms first we have three carbons on the
left
so we're going to put in 3 in front of
co2
next we're going to balance the hydrogen
atom so we have 8 on the left 2 on the
right we know 4 times two is eight
and then finally we need to balance the
oxygen atoms
we have six oxygen atoms and the three
co2 molecules four on the right from h2o
that's ten
ten divided by two is five so we need to
put a five in front of o2
so now the reaction is balanced
okay
let's make a list of what we know we
have 30 grams of propane
now it's burned in air so we can assume
that we have an excess amount of oxygen
whenever you have two reactants you need
to identify what's the limiting reactant
and what's the excess reactant you
always want to use the limiting reactant
to calculate your theoretical and
percent yield
never the access reactant
now
the equation for percent yield
it is the actual yield
the divided by the theoretical yield
times 100 percent
so
you need to know the difference between
the actual and the theoretical yield
the theoretical yield is what you
calculate it's the maximum amount of
product that you can get so if your
reaction is 100 efficient
you know the amount that you get is the
theoretical yield
in actuality you know you know you never
get 100 yield it could be 80 70
you know
it's never perfect
the actual amount that you get in
reaction that's your actual yield so the
actual yield is usually given to you in
a problem unless they want you to solve
for it
and the actual yield usually is the
grams of product you see how we want to
find the percent yield of
co2 that means that
we need to use the actual yield of co2
and the theoretical yield they gave us
the actual yield of co2 they said you
know we
you know 70 grams of carbon dioxide was
produced that's how much we actually got
in the experiment
so we know the actual yield
what we don't know is the theoretical
yield the most that we can get from the
experiment
so let's calculate the theoretical yield
the maximum amount of co2 that is
possible to get
so we're going to start with the 30
grams of propane
we're going to see how many grams of co2
can be produced at most from 30 grams of
c3h8
so let's start with 30 grams of c3h8
the first step is to convert to moles
using molar ratio
so the molar mass of c3h8 which we get
from the periodic table
it's about 44 grams
per 1 mole
you know carbon has an atomic weight of
12
so 12 times 3 is 36 hydrogen is about
one and there's eight of them so 8 plus
36 you get 44.
now you want to do this in such a way
that the grams of c3h8 they cancel
the next thing you want to do is um
you want to
you want to change the substance you
want to convert from
moles of c3h8 to moles of co2
whenever you want to change from one
substance to another substance you need
to use the molar ratio and the molar
ratio it's a one one to three
for every one mole of c3h8 that burns in
the reaction three moles of co2 are
produced
and
since we have moles of c3h8 on the top
left
we need to put that on the bottom right
so that they cancel
and therefore the moles of co2 will go
on top
and then we simply add the numbers
there's a one in front of the c3h8 so
we're going to put that here and there's
a 3 in front of the co2 so we'll put
that there as well
so moles of c3h8 they cancel now
the last step is to convert from moles
back to grams whenever you convert from
grams to moles or moles of grams you
need to use the molar mass in the
periodic table which is the same as the
atomic mass carbon has an atomic mass of
12
oxygen is 16 and there's two of them so
that's 32 plus 12 that's 44.
now because we have
moles of co2 on the top left we need to
put moles of co2 on the bottom right we
want those units to cancel
so therefore grams of co2 has to go on
top
and we said the molar mass of co2 is 44
so that means that there's 44 grams of
co2 per one mole
so now we can do the calculation
so the numbers on top you multiply and
you divide by the numbers on the bottom
so 30 divided by 44
times 3
times
44.
so this gives us
a theoretical yield of 90 grams so what
this means is that if all of the 30
grams of c3h8 all of that reacts
the most that we can get the maximum
amount of co2 that can be produced in
this reaction is 90 grams of co2
so that's what the theoretical yield
means the the maximum amount that you
can get
well in this reaction we only got 70.
so now that we have the theoretical
yield we can find the percent yield
so the percent yield is going to be
70
divided by 90 is always the smaller
number divided by the large number times
100 percent
and that is going to equal
77.7
or 77.8 percent so that's the percent
yield of this reaction that's how you
find it
so that's it that's all i got for you
today
