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AP Chemistry 2.1 Rearrangements and Reorganization of Atoms. What is the percent yield for this reaction?

Language:
English Language

Transcript

00:04

Here’s your Shmoop du jour, brought to you by decaffeinated coffee, also known as a cruel, [Man drinks decaf coffee]

00:09

cruel joke.

00:10

Here’s today’s question:

00:12

In the early 1900s, scientist Ludwig Roselius popularized the use of benzene (C6H6) to decaffeinate

00:21

coffee, which led to the production of Sanka®.

00:24

Although this process is no longer used today, as, benzene is carcinogenic, this compound

00:30

has many other important industrial uses.

00:32

In the following reaction, 25.0 g of benzene, C6H6, reacts with excess HNO3, which results [Chemical reaction appears]

00:41

in 19.7 g C6H5NO2.

00:45

What is the percent yield for this reaction?

00:49

And here are your potential answers:

00:52

The first thing we should do is figure out what we’re actually being asked. [Student raises hand to answer question]

00:56

We have a feeling there’s a whole lot of fluff in this one.

00:59

We don’t care about this Ludwig van Benzthoveen guy and whatever company his blasphemously [Beethoven sipping coffee]

01:04

undercaffeinated coffee created, we just want to know about the reaction.

01:09

So the important part of the question is just the last two sentences, the equation, and

01:13

the answers.

01:14

Boom, boom, boom, cut all that unnecessary stuff out of your life. [unnecessary part of question disappears]

01:17

Okay, so it’s asking us about the percent yield and giving us grams going into and out

01:22

of a reaction.

01:23

And by grams, we mean the unit, not the delicious cracker. [Man takes cracker and person uses wand to make it disappear]

01:27

Yeah, we were disappointed, too.

01:28

Because this is a question involving specific amounts of chemicals reacting, before we even

01:32

think about how to solve it, we’re going to convert everything to moles.

01:36

All of our given values are in grams, so let’s convert from grams to moles by using the molar

01:40

mass.

01:41

Which means we have to calculate the molar mass.

01:42

The molar mass of benzene is 6 times the atomic weight of Carbon, 12.01 grams per mole from

01:46

the periodic table, plus 6 times the atomic weight of Hydrogen, 1.01 grams per mole, which

01:52

gives us 78.12 grams per mole.

01:56

The molar mass of C6H5NO2 is 6 times the atomic weight of carbon plus 5 times the atomic weight [molar mass appears]

02:01

of hydrogen plus the atomic weight of Nitrogen, 14.01 grams per mole, plus 2 times the atomic

02:08

weight of Oxygen, 16.00 grams per mole, which gives us 123.12 grams per mole.

02:14

Whew.

02:15

Still with us?

02:16

Now, let’s calculate the moles of each substance so we can stop talking about weight so much. [Benzene and nitrobenzene on balance scales]

02:20

It’s making the molecules self-conscious…

02:24

To find the moles of each substance, we take the mass in grams and divide by the molecular

02:29

weight.

02:30

This is 25.0 grams over 78.12 grams per mole for benzene, giving us 0.320 moles of benzene. [moles calculation appears]

02:38

For C6H5NO2, it’s 19.7grams over 123.12 grams per mole giving us 0.160 moles of C6H5NO2.

02:48

Time to mop your brow. [Worker wipes away sweat from brow]

02:50

We did it.

02:51

Wait… what was the question?

02:53

Oh right.

02:54

Percent yield.

02:55

And that’s not a matter of how often one chemical let’s another one pass at an intersection. [Car stops at a yield sign]

02:59

It’s how much product you got out of a reaction.

03:03

Specifically, how much product you actually get as a percentage of what you could have

03:06

gotten if everything went perfectly.

03:08

Because, like life, things in chemistry don’t always go perfectly. [Scientist eating a cookie]

03:14

In this reaction, we see the same number of benzenes and C6H5NO2 molecules on either side

03:18

of the reaction, so each time we use a benzene we make a C6H5NO2.

03:24

This means we could ideally make as many moles C6H5NO2 as we had moles of benzene to start

03:30

with, which we calculated to be 0.320.

03:32

To calculate the percent yield, we take the moles we actually got over the moles we could [percent yield equation appears]

03:37

have gotten, 0.160 over 0.320, and multiply by 100.

03:41

This gives us a 50 percent yield for the reaction.

03:44

Which isn’t a great result, but it is our answer.

03:48

If your lap supervisor is disappointed, you can offer him a cookie. [Man offers supervisor a cookie]

03:50

Or 20.

03:51

But no more.

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