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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