Name: Worked example: Calculating mass percent | AP Chemistry | Khan Academy
Uploaded: 2020-03-27T17:38:12.000Z
Duration: 5 min 38 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

I have the molecular formula for glucose.

And so let's just say that I had a sample

can you figure out the percentage of carbon

And as a hint, I've given you the average atomic masses

All right, now let's work through this together.

Now, the reason why the amount of glucose doesn't matter

should be the same regardless of the amount.

Because every glucose molecule has six carbon atoms.

So we could say, what is going to be the mass

And once again, the reason why it's six moles of carbon

divided by one mole of glucose is because this,

every molecule of glucose has six carbons.

So it's going to be six times as many carbon atoms

And notice the numerator will be just left with grams.

we have six carbons, 12 hydrogens, and six oxygens.

So it's going to be the mass of six moles of carbon,

12 moles of hydrogen, and six moles of oxygen.

So it's going to be what we just had up here,

which is going to be 1.008 grams per mole of hydrogen.

which is going to be 16.00 grams per mole of oxygen.

And the good thing is, down here, the units cancel out,

so we're left with just grams in the denominator.

We're gonna end up with grams in the numerator,

grams in the denominator, the units will cancel out,

and we'll get a pure percentage at the end.

I'm just going to do the pure calculation first,

and then I'm gonna worry about significant figures.

and then we have plus six times 16 is 96.00,

if we're just thinking about the pure calculation,

before we think about significant figures,

not even think about significant figures.

if we were just doing this blue calculation here,

that should only have four significant figures,

it would have gotten us to the hundredths place,

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Worked example: Calculating mass percent | AP Chemistry | Khan Academy

assume

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matter