Tutor Hotline, Melissa Maribel here, how can I help you?
Hi Melissa, my name is Anakaren
and my question is question number 12.
Which says, how many moles are in a
5.35x10^22 molecule sample of Na2SO4
I've already started to work on this problem but
I'm not sure if it's correct and any advice and help would be appreciated.
Ok, so let's work on this question together
and just figure out exactly what to do
my advice on
any tips or tricks I have to always get
these questions right especially for stoichiometry. So let's start that.
For any word problem that you see
Always identify what you're given and what you're finding.
So that's step 1.
We will literally just see, what is this question giving us
to start with and then what are we finding overall.
So what this question is saying is
we are given how many molecules this sample has.
So that's what we're starting with.
And then we're asked to find the amount of moles overall in this compound.
So we know that we're going from molecules to moles of the same exact compound.
That is extremely important to see
one are we changing the compound are we keeping it the same
Because that helps us figure out how many steps this question will take.
So the next step is to pretty much just identify
What we have to go to next
So are we going, like how many conversion factors are we going to need?
That's the next step.
And let's go over just the different types of conversion factors
you will most likely see for a stoichiometry problem.
Alright, for step 2
We said that we're going to determine which conversion
factor or factors we have to use.
So the first common one is known as molar mass.
What molar mass allows you to do is it allows you to go from
grams to moles or the opposite way
from moles to grams. Looking at this question
We're not given grams whatsoever so there's no need
to convert anything to grams so this one is out.
We're not using molar mass overall.
Let's do another one, let's see what other conversion factor we could possibly use.
Our other type of conversion factor is when we're doing a
mole to mole ratio.
For this we are only going from
moles of one compound and then we're changing the compound.
So if we're changing the compound we will use a mole to mole ratio.
However here we said we're
keeping the exact same compound so nothing is changing.
So once again now mole to mole ratios
We're not using that, that's not needed whatsoever.
So nope, that one we're not using that either.
What's the next one? So another
conversion factor that's possible is
going from moles to liters.
This one, if you haven't learned it already you're going to learn it in a later section.
But just know that we are
able to go from moles to liters or liters
to moles and this question is not telling us
anything about liters so we're not using that at all.
So ok that one is out as well.
And the last one we are going to see is
the actual one we will be using.
So, this is our other conversion factor, Avogadro's number.
And the Avogadro's number
there's a lot to it, however what I want you to focus on
because Avogadro's number has atoms
molecules, particles, formula units and all of that
really what this is saying is, if you see
one of these units either atoms
molecules, particles, or formula units
you will use Avogadro's number. So in this case
because of the word molecules, and that's what
I was given, I know instantly that I have to use Avogadro's number.
Plus Avogadro's number allows you to go from
molecules to moles
and vice versa we could also go from moles
to molecules. So that's how I know exactly to use
this conversion factor and this conversion factor only.
And then just note, remember what Avogadro's number is.
Avogadro's number is one that mole is equal to 6.022
times 10^23 molecules.
So now let's just set everything up and do the math.
So our next step is to convert our units
and check our amount of sig figs because
we always want to round to the correct amount of sig figs.
So what I'm gonna do is I always want to start off with
whatever I'm given. Alright so
whatever I'm given initially and that was the molecules
we initially were given our molecules so that's what I start with.
And then what I'm going to end with is whatever I'm finding.
So what I want to do right now is I'm only gonna use
one conversion factor which was our Avogadro's number.
We want to align the units so they can cancel.
Meaning our molecules will be on the bottom
and I'll show that here.
So our molecules will be on the bottom because
we want those to cancel. So if you don't
know when to use the conversion factor, sometimes you're going
to see it's flipped, sometimes you're going to see that you're
multiplying or dividing, it's all dependent on the units.
Alright, so whatever we want to do
whenever we look at this, you always want to cancel out the units.
So that's how I knew to cancel out the molecules
that Avogadro's number has to be on the bottom.
And that's exactly what would happen here, is Avogadro's number would cancel out
because our molecules would cancel out and then we're going
to be left with just our moles. So all we have to do is divide
these two values and we'll get our final answer
which is if you note it's going to be
rounded to 3 sig figs. The reason why it's
rounded to 3 sig figs was because our initial value which was given only
has 3 sig figs if you notice here
5.35, there we have
3 sig figs right, there's 3 sig figs
and then here we're also going to see our final answer has 1, 2,
3 sig figs. If you want to get your question answered
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