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 98x2. 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.