In this video you are going to learn how to calculate molar mass and how to
convert from grams to moles and moles to grams like a boss. All right, let's do this.
Hello hello Melissa Maribel here and I help students like you understand what you
just learned in class, so you stress less and you graduate faster. If you're
serious about passing Chemistry, hit that subscribe button. On to the two types of
masses used in chemistry, atomic mass and molar mass. Both of these refer to the
mass of an element but they have different units. Atomic mass has amu or
atomic mass units. Molar mass has grams per mole. That molar refers to the moles
on the bottom and the mass refers to the grams on top. All right so let's say if
we just want the individual element of, let's say carbon. If we only want that
individual elements molar mass you refer to the periodic table and you'll see
that carbon has a 12.011 mass. We'll add the grams per mole and that's what makes
it molar mass. Now if we wanted to figure out the molar mass of an entire compound,
let's say CO2 or carbon dioxide. We would have to add up each individual element's
molar mass. Take carbon, we said it was 12.011 grams per mole. We'll add that to
2 times 16 because oxygen has 2 oxygens within that CO2 compound. 2 times
16 that'll give us 32. Adding the 32 plus the 12.011, we will get a total molar
mass of 44.011 grams per mole. Now that we've gotten molar mass figured
out, remember conversion factors? Yes, I hope. So a conversion factor is a number
that has two different units. Molar mass has grams per mole. So we know that
that's going to be used as a conversion factor. Whenever you're going
from grams to moles or moles to grams that is your clue to use the molar mass
as a conversion factor. Let's get into our example and we'll convert grams to
moles. How many moles are in 5.3 grams of iron chloride? Always identifying your
given, what you're finding, and our plan. Your given is 5.3 grams iron chloride.
We're going to moles of iron chloride. Our plan is just converting from grams to
moles and as we stated, you always use molar mass as a conversion factor
whenever you're going from grams to moles. Step one is to find our molar mass
of iron chloride. We'll take the individual masses of iron which is just
55.85. There are two chlorine within this entire compound, so
we have to multiply by 2, times the one individual chlorines molar mass. So
multiplying that out and adding by 55.85 we will get a molar mass
of 126.75 grams per mole. All right let's set this up. Always starting with
your given, aligning your units across from each other so they can cancel and
we'll be left with moles of iron chloride. Our final answer is
0.042 moles of iron chloride. Let's go in reverse going from moles to
grams. What is the mass of 0.30 moles of magnesium nitrate?
Your given is that 0.30 moles magnesium nitrate. You are
finding grams of magnesium nitrate, and our plan is going from moles to grams.
Once again we'll use our molar mass as our conversion factor. Step one is to
find your molar mass of that magnesium nitrate. Magnesium has a mass of 24.31
but your nitrogen, there's actually two. I want you to think of this subscript as
distributing to each other's subscript. So in our case we'll have two different
nitrogen and 3 times 2, so 6 different oxygen. We'll multiply 2 times
the mass of just one individual nitrogen and then 6 times the mass of one
individual oxygen. Once we add this all together we'll get
148.33 grams per mole. Let's set up our conversion and we'll
have 0.30 moles on top of magnesium nitrate. Aligning your moles
across from each other, those moles would then cancel and we'll get two grams of
magnesium nitrate. Multiply straight across. Our final answer is going to be
two significant figures because our given was only two sig figs.
So our answer is 44 grams magnesium nitrate.
(Music)
Did you convert using molar mass like a boss? I've linked practice problems below.
Also, I'm doing free live tutoring, sign up today to reserve your spot. That's all
for today, LIKE, subscribe and I'll see you next time.