In this video you are going to learn how to find a compound's molecular formula.
Alright let's do this.
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So what is an empirical formula versus a molecular formula? Great question, an
empirical formula is an educated guess or an approximation as to how many
elements are within that compound. It is by no means the exact chemical formula.
The exact chemical formula is the molecular formula. So in order to figure
out a molecular formula, you first have to figure out your empirical formula.
Let's try one.
Nicotine has a molar mass of a 162.1 grams per mole
and a composition of 74.0% of carbon, 8.70% of hydrogen
and 17.30% of nitrogen. Find the molecular formula.
To find the molecular formula you first
have to figure out what your empirical formula is. So our first step is to
change our percentage to grams. To do so note that if we were to add up all of
these percentages it would be out of 100%. So because it's out of 100%
there is no need to multiply or convert anything. You are
literally changing your percentage to grams. These will be our new values that
we'll use in the next step. Step two is to convert your grams that we found in
our previous step to moles. When converting to grams to moles we use
molar mass of each element to get to moles. So in our case we'll have the
seventy four point zero grams of carbon, align that with the molar mass of carbon
which is 12.01 grams. Those grams would cancel and then we'll get our
6.1615 moles of carbon. We'll do the same thing for hydrogen except
aligning hydrogen's molar mass on the bottom and then we'll
divide and you get 8.613 moles of hydrogen. And lastly we'll
do the same exact thing for nitrogen except nitrogen's molar mass is
14.01. Divide these values and you get 1.234 moles
of nitrogen. The next step is to then divide by the smallest number of moles.
Which in our case will be the nitrogen. Divide each one of these values and your
moles would cancel and we're left with the new subscripts of our empirical
formula. Now we have our empirical formula, those subscripts that we found
in the previous step are as shown and now it's on to figure out our molecular
formula. So the next step is to figure out the molar mass of this empirical
formula. Start with your carbon which we have 5 of them and multiply by the
individual molar mass of carbon. We'll get 60.05. Do the
same for hydrogen. Now there are 7 times that 1.01 and you
get 7.07. But there's only 1 nitrogen so you don't even have
to really multiply. It's 1 times 14.01 which is itself.
We'll add all these values together and your molar mass of your empirical
formula gives you 81.13 grams per mole. Your next step
is to divide. You will be dividing your molecular formulas molar mass, divided by
the empirical formula's molar mass. In the initial question you were given the
molecular formula's molar mass. You will always be given that because there is no
other way to figure out the molecular formula without the molecular formula's
molar mass. We found our empirical formula's molar mass and a nice little
trick for you to remember that molecular formula's molar mass goes on top and the
empirical formula's molar mass goes on the bottom is, "please oh please don't
forget about ME." ME molecular formula on top, empirical formula's molar mass on the
bottom. So we'll divide those two values in our case and these grams per mole
will cancel and you get a whole number of 2.
Our final step is to multiply that whole number which was 2 that we found in
the previous step, to each subscript of your empirical formula. So in the black
you'll see our empirical formula. We're multiplying by two that we found before.
And finally we get our molecular formula of C 10 H 14 and N 2. This is the exact
chemical formula of nicotine.
Let's test out what you just learned.
I know these questions are a lot of steps but the more you practice the
easier it's gonna become and you're gonna remember all the little steps.
So if you have to rewatch this video, if you have to just keep practicing over and
over, you will pass chemistry, it's really just about practice. But if you need some
more help check the description box where you can reserve your spot today
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concepts in Chemistry. make sure you LIKE subscribe and I'll see you next time.