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
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