ShmoopTube

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

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cruel joke.

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Here’s today’s question:

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

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coffee, which led to the production of Sanka®.

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

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has many other important industrial uses.

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

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in 19.7 g C6H5NO2.

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What is the percent yield for this reaction?

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And here are your potential answers:

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

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

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

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

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

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the answers.

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

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

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of a reaction.

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

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Yeah, we were disappointed, too.

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

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

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

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mass.

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Which means we have to calculate the molar mass.

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

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

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gives us 78.12 grams per mole.

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

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

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

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Whew.

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Still with us?

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

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It’s making the molecules self-conscious…

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

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weight.

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

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

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

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We did it.

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Wait… what was the question?

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Oh right.

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Percent yield.

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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]

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

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

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gotten if everything went perfectly.

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

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

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

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

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with, which we calculated to be 0.320.

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

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

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This gives us a 50 percent yield for the reaction.

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

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

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Or 20.

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But no more.

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