Stoichiometry: Converting Grams to Grams


All right let’s jump right into our
stoichiometry problem our problem states, how many grams of calcium hydroxide are
needed to react with 41.2 grams of phosphoric acid? and they give us our
balanced equation. So like always let’s identify what we’re given and what we’re
finding. Our given was the 41.2 grams of phosphoric acid and then what
we’re finding are, how many grams of calcium hydroxide we have. So let’s build
a plan. So we’re gonna start off with our given unit and our given unit was grams
of phosphoric acid. So that’s where you’re always going to start by the way,
that’s why we stated what our given was and then you’re gonna end with what
you’re finding. So we’re starting with grams of phosphoric acid and then we’re
gonna make our way to moles of phosphoric acid. And then how we’re gonna
do that is by using the molar mass. Whenever we’re going from grams to moles
or moles to grams you use the molar mass as your conversion factor. Now continuing
from the moles of phosphoric acid, we’re looking for a completely different
compound. So right then and there, that tells us that’s a mole to mole ratio and
where we get a mole to mole ratio is from our balanced equation, those coefficients
in front. So from moles of phosphoric acid, I’m going to get to moles of
calcium hydroxide and then lastly we’ll be able to use the molar mass of calcium
hydroxide to end with what we’re finding which is grams of calcium hydroxide. So
let’s set this up. Let’s start off with the grams of phosphoric acid and then to
get from grams to moles we have to use the molar mass of phosphoric acid. So
let’s figure out what the molar mass of phosphoric acid is. So starting with
hydrogen, we’re going to take its individual mass and multiply that by
three since there are three hydrogen’s within that compound. So hydrogen has a
mass of 1.01, I’m going to multiply that by 3 and we’ll get 3.03. Next phosphorus has a mass of 30.97,
oxygen has a mass of 16 but there are four oxygen within phosphoric acid so I’ll
multiply that by 4 and then after multiplying that by 4 we’ll get 64.
Next add all of these masses up and that will give us the molar mass of 98 grams
per mole. This is what we’re going to use to go from grams of phosphoric acid to
moles of phosphoric acid. So from here what’s gonna happen is our grams of
phosphoric acid would then cancel and we’re now at moles of phosphoric acid
and we want to do a mole to mole ratio to change the compound to calcium hydroxide.
So let’s go back to our balanced equation because that’s what we’re gonna
find our multiple ratio looking at the coefficients in front of each
compound. So, we have two moles of phosphoric acid for every three moles of
calcium hydroxide. So this is what I’m gonna put on top I’ll put the three
moles of calcium hydroxide and on the bottom aligning those units, we always
want those units of moles and moles to cancel, so two moles of phosphoric acid
would be placed on the bottom and I’ll cancel those out. Now we’re finally at
moles of calcium hydroxide. Last step is to find the molar mass of calcium
hydroxide because remember molar mass allows us to go from moles to grams. So
let’s figure out what our molar mass is of calcium hydroxide. So once again
taking each individual mass of each element we’ll see that calcium has a mass
of 40.08. Oxygen is typically 16 however there are two
oxygens within this compound so I’ll multiply that by 2 to get 32. Hydrogen is
typically 1.01. Once again we’re going to multiply that by two
since there are two hydrogens within calcium hydroxide and then that gives us
2.02. Add all of these masses up to get the final molar mass of calcium
hydroxide which is 74.1 grams per mole. So we’re gonna put the grams on top of
calcium hydroxide since, remember that’s what we’re finding. So whatever we’re
finding that’s going to be our very very last step, that’s going to be
placed on top and then we’ll put our moles of calcium hydroxide across from
each other so they can cancel. And finally we’re at grams of calcium
hydroxide, make sure to multiply straight across and divide by 98×2. So
we’ll get our final answer of 46.7grams of calcium hydroxide.
I know stoichiometry is a huge part of chemistry, so that’s why I’ve made so
many videos on stoichiometry already so make sure to check those out plus I even
designed detailed notes for you guys to see all the different possibilities like
going from liters to atoms or let’s say from grams to grams of a completely
different compound. So I placed all these different types of examples so you can
practice and make sure to do extremely well on your exams. Alright guys make
sure to check out the description box for all of these awesome resources and
like this video and subscribe if I’ve helped you in any way. Keep going guys,
love you so much. See you in the next video.

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