a

Mole Ratio Practice Problems

okay so now I'm going to do a whole

bunch of mole ratio problems a whole

bunch so you can really get a handle on

these they're super important for

stoichiometry for understanding

stoichiometry okay each problem I'm

going to do it two ways first I'm going

to treat the equation the chemical

equation kind of like kind of like a

recipe all right where we've got our

ingredients and then we've got the step

we're baking this method really makes

sense like you'll understand what you're

doing but it requires a little bit of

thumb then I'll solve each probably

using a conversion factor method the

conversion factor method doesn't require

any thought but it doesn't make any

sense so it's really easy to get in the

habit of only using the conversion

factors running through the math but

having absolutely no idea what you're

actually doing or why you're doing it

okay so that's why I'm going to do each

of these problems two different ways

okay

so let's get started here is the first

equation I'm going to be working with

two moles of h2o water makes two moles

of hydrogen gas and oxygen here if

there's no coefficient in front of the

oxygen in front of one of these

chemicals we know that it's really one

that it means one mole so hey if it

helps you go ahead and write that one

there's our equation here is the

question how many moles of o2 of oxygen

will be produced from six point two

moles of water okay so first of all I

want you to think of this like a recipe

and right now it's saying that we start

with two moles of this and that gives us

two moles of this and one mole of this

but we're not talking about starting

with two moles of h2o we're talking

about starting with 6.2 moles of h2o

okay so this is just like when you're

cooking and you've got a double or

triple or quadruple a recipe what do we

have to do to this recipe so that

instead of starting with two moles of

water we start with six point two moles

of water okay we have to multiply

everything in this equation by something

that's going to get us 6.2 moles of

water

we can figure this out a couple lists

first thing I can do is I can do 6.2

which is this divided by two which is

that and that gives me 3.1 okay that

like gives me the factor and what I mean

by that is that we take each one of

these numbers and we multiply it times

three point one two times three point

one and now we get six point two moles

of this okay so now we're starting with

six point two instead of two we multiply

this by its 3.1 now we'll also want to

multiply the number of H two times three

point one so now we're getting six point

two moles of H 2 and we want to take

this number which is one times three

point one multiply it there and we get

three point one moles of o2 so once

again we treat this like a recipe in the

kitchen and we're doubling or tripling

instead of multiplying by two or by

three or by 4 we multiply everything in

this recipe by 3.1 we get six point two

here so that's what we're starting with

then we get six point two moles of h2

and we get three point one moles of o2

and that kind of makes sense right

because there's this two to one ratio of

water to oxygen we start with two of

these we get one of these so this is

half of what we have here okay so if we

start with six point two we multiply

everything by three point one and we get

six point two here and this three point

one is half of what we had over here

okay so that's how we can do this

equation treating it like a recipe in

the kitchen now let's look at how we can

do this using conversion factors okay

what we're going to do is we're going to

start with six point two moles of h2o

now what I want to do is I want to write

a conversion factor that tells me the

relationship

between the moles of water and the moles

of oxygen okay we can sum it up with

this it tells us for wherever two moles

of h2o I get one mole of o2 two here one

here okay we get this relationship and

what I can do is I can use this to write

a conversion factor there are two

different conversion factors I can get

from this I can write two moles of h2o

over one mole of o2 okay that's one or I

can flip it I can do one mole of o2 over

two moles of it so of course I'm just

getting the numbers from the chemical

equation two and one so I've got these

two conversion factors that are flips I

could use either one of them they're

both equally good the one that I want to

choose though is the one that I can

multiply by this and cancel out moles of

h2o okay that's going to be the one that

has h2o on the bottom because it's on

the top over here so I'm not gonna use

this one I am going to use this one I'm

going to multiply it here and now I have

h2o on the top h2o on the bottom so they

cancel out and when I do this math I'm

going to do six point two times one

divided by two equals three point one

moles of o2 and check it out I use a

conversion factor method and I get the

exact same answer that I got when I use

this like a recipe okay so the recipe or

the conversion factor both give the same

number okay let's go on to the next one

okay so how many moles of h2o will be

required to make 19.2 moles of o2 let's

treat this like a recipe first okay

right now this is a recipe for one mole

of o2 what are we gonna have to do to

this recipe double it triple it

quadruple

is what I mean what are we going to have

to do to this recipe to make it a recipe

for nineteen point two moles of o2 well

there's a one right here one mole of o2

in the recipe so it shouldn't come as a

shock that we have to multiply the

recipe by nineteen point two and now if

we multiply it by nineteen point two it

will become a recipe for nineteen point

two oh two times nineteen point two and

this is going to give me thirty eight

point four moles of h2 multiply this by

nineteen point two because just like

when you're cooking right you got to

multiply everything in the recipe by the

same number and this is going to give us

thirty eight point four moles of h2o

this is what happens to the recipe when

we size it up to cook for nineteen point

two moles of o2 so the answer is how

many moles of h2 are going to be we're

going to need thirty eight point four

moles of h2o that's the recipe method

okay

now let's look at the conversion factor

method just as before we realize the

relationship between water and oxygen by

using the chemical equation here is two

of these two one two these so we can get

these two conversion factors yeah one

mole of o2 to two moles of h2o or two

moles of h2o to one mole of o2 I'm going

to start here with nineteen point two

moles of o2 and I want to be able to

multiply this by the conversion factor

that's going to cancel out my moles of

o2 so I'm going to choose this one here

because it has moles of o2 on the bottom

cancel out cancel out I do 19 point two

times 2/1 equals thirty eight point four

moles of h2o

I know it's moles of h2o because that's

the unit that I have remaining

okay so sometimes people ask why are you

doing it both ways why are you doing the

acquit the recipe way and then the

conversion factor way okay the reason is

because for me the recipe way makes a

lot more sense but teachers and

textbooks are in love with conversion

factors they love them they can't get

enough I don't think the conversion

factors make any sense at all all right

but just because they're like commonly

used your teachers going to ask you to

do it your textbooks going to ask you to

do it that's why I'm doing them too but

what I want you to focus on is actually

why we're doing it the way we multiply

things and get the answers here with

pretending it's a recipe I want you to

focus on this and you understand what's

going on and as you can always see the

answer that we get from the conversion

factor method is the same as the answer

that we get when we're treating it like

a recipe of multiplying everything by

some number okay

all right let's move on to the next

question so two moles of h2s hydrogen

sulfide combined with three moles of o2

oxygen to make two moles of so2 and two

moles of h2o using this equation we're

asked how many moles of o2 are needed to

combine with 8.4 moles of h2s okay what

do we have to do with our recipe to cook

with 8.4 moles of h2 s instead of two

moles of h2s what we're going to want to

do is we're going to multiply this by

some number that's going to give us 8.4

how can we figure out what that is

well you might sort of be able to do it

in your head or you can do 8.4 divided

by 2 and that's going to give us four

point two so 4.2 is the number that

we're going to have to multiply

everything by so that we can move from

two moles of this to a recipe that uses

8.4 moles of this okay so everything

we're going to multiply it by 4.2 okay

so here we have

three times 4.2 and that's going to give

us 12 point 6 moles of o2 to two 38.4 to

twelve point six all right let's see how

we can do this using conversion factors

we'll start with eight point four moles

of h2s and multiply that by one of these

two conversion factors these conversion

factors are just telling us the

relationship that we have here of two

moles of h2 s to three moles of o2 and I

can use this relationship to make these

two conversion factors which one do I

want to use I want to use the one that

has h2s on the bottom so that it cancels

out so I choose this one cancels F

cancels out and this is going to give me

what a shocker twelve point six moles of

o2 one moles of o2 because it's the unit

that's left here okay that's how we can

do it treating it like a recipe here's

how we can do it using the conversion

factor here's an XML ratio problem look

maybe you're getting the hang of these

maybe you feel great turn the video off

then and move on to the next thing

but if you still like some or practice

hey let's party all night long with mole

ratio problems we'll keep doing them

until it feels really comfortable and

you can and you're sure that you could

like do any question it's thrown at you

okay starting with 9.2 moles of o2 how

many moles of h2s will you need and how

many moles of so2 will you get okay

so starting with 9.2 moles of o2 right

now our equation is written to start

with three moles of o2 so what are we

gonna have to do we have to find out how

to double triple quadruple whatever we

want to do to this to get it up from

three moles of o2 to nine-point

two moles of objects what am I going to

have to multiply it by to turn this 3

into a 9.2 I can figure that out by

doing nine point two divided by three

and I'm going to get three point one so

three times three point one is going to

give me nine point two I'm kind of round

in here all right if you do the math

it's not exactly right okay times three

point once going to give me nine point

two moles of o2 and how many moles of h2

s am I going to need well I'm going to

have to multiply everything in the

equation by the same amount so times

three point one here is going to give me

six point two moles of h2 s so that's

how much h2s I'm going to need to

combine with my nine point two moles of

o2 and then how many moles of so2 will I

get I take my two moles of so2 and I

multiply that by three point one as well

I've got to do the thing same thing the

whole equation is when I get six point

two moles of so2 so that's how we do

that let's look at how we do the

conversion factors you guys are really

probably getting the hang of this now so

I'm not going to show all the conversion

factor steps we start with nine point

two moles of o2 times what is our

relationship between Oh two and h2s we

have three moles of o2 - two moles of h2

s so I want to write that as a

conversion factor with o2 on the bottom

so that cancels out so three moles of o2

- two moles of h2 s moles of oxygen up

here cancel out little action down there

cancel out and I get the same answer

that I got here I get nine point two

moles of h2s and now let's just show how

we can do this with the moles of so2

it's going to be the exact same thing

I'm not even going to like get rid of

this thing it's all canceled out here

I'm going to multiply it by equation

that show

is a conversion factor that relates

oxygen here and so2 here okay so again

three moles of this to two moles of this

so three moles o2 goes on the bottom and

two moles so2 goes on the top cancel

this is how to already this is cancel

that and I'm going to get after I do the

math nine point two times two divided by

three equals six point two moles of so2

sometimes I write moles or the knees

sometimes ml well because sometimes I

get lazy okay so that's that let's do

two more if this feels good turn it off

and move on so here we have an equation

touch about propane burning which is

c3h8 nothing in front of this so we can

put a 1 here one mole of this plus five

moles of o2 gives me three moles of co2

and four moles of h2o my question here

asks me how many moles of oxygen are

needed to react with 7.2 moles of

propane what do we have to do to this

equation to size it up so that we're

starting with 7.2 moles of propane right

now we're starting with one mole of

propane okay so this isn't the hardest

thing in the world we're going to have

to multiply this whole equation times

7.2 and that's going to give us seven

point two moles of propane how many

moles of oxygen turn eated well we have

to multiply it by 7.2 as well and that

math is going to give me 36 point 0

moles of o2 let's look at the commercial

factor method we're going to start with

seven point two moles of c3h8 and we're

going to multiply that by a conversion

factor that says one mole of C 3 H 8 to

5 moles of o2 which is going to be on

the top which is going to be on the

bottom this is on the top so we want

this in a conversion factor to be on the

bottom so I say one mole I'm getting it

from right here

one mole c3h8 below five moles

Oh - this cancels out its on top this

cancels out its on the bottom and it

gives us seven point two times 5/1

equals 36 point 0 moles of oxygen

okay one more our last one how many

moles of propane are I needed to make

thirteen point five moles of co2 in

order to do that how much o2 will be

needed okay what do we have to do to our

equation right now in order to make it

thirteen point five moles of co2 instead

of three moles of co2 we have to

multiply it by some number that's going

to move it from three up to thirteen

point five and what I can do is I can do

13 point 5 divided by three and that's

going to give me four point five so I

can multiply this by four point five and

that will turn my three into a thirteen

point five moles of co2 so then when I'm

talking about how many moles of propane

am I going to need I multiply it by the

same amount and I get one times four

point five four point five moles of C 3

H five and then I'm asked how much o2 am

I going to need well I have to do the

same thing to all the pieces of my

recipe so five times four point five is

going to give me twenty two point five

moles of o2 this is what happens to my

recipe when I increase the size of it

let me just show you these really

quickly I'll start with thirteen point

five moles of co2 times a conversion

factor that talks about the relationship

between propane c3h8 and co2 here co2 is

going to be on the bottom so that it

cancels out so three moles of co2 on the

bottom one mole

of c3h8 on the top 13.5 times 1/3 is

going to give me four point five moles

of c3h a the number is the same even

though the process is different Oh what

did I forget to do I forgot to cancel

these out or I could I'm not going to

get rid of it I keep it there or to find

out how much o2 I'm going to need well

what is a relationship between co2 and

o2 it's 3 to 5 so 3 moles co2 on the

bottom and 5 moles of o2 on the top

these cancel out 13.5 times 5/3 equals

22.5 moles o2 same answer regardless of

whether I do it the recipe method or the

conversion factor method if you made it

all the way to this end of this video

you party with me all night long on mole

ratios you're a rock star but more than

anything I bet this really makes sense

and I bet you can tackle any mole ratio

problem that's thrown at you feel free

to use the recipe method or the

conversion factor method unless your

teacher says that you have to use a

conversion factor method then go ahead

and use it but know in the back of your

mind what is actually going on and why

you're multiplying by what you're doing

and all that sir stuff all right

good luck