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  • - [Instructor] We are asked to calculate

  • the number of moles in a 1.52 kilogram sample of glucose.

  • So like always, pause this video

  • and try to figure this out on your own

  • and this periodic table of elements will prove useful.

  • All right, now if we're trying to figure out

  • the number of moles, remember, mole is really,

  • you can view it as a quantity of something.

  • If I said a dozen of something,

  • you'd say oh, that's 12 of that thing.

  • If I say a mole of something,

  • I'm saying that's Avogadro's number of that thing.

  • And so we have a 1.52 kilograms sample

  • of our molecule in question, of glucose

  • so if we can figure out the mass per mole,

  • or another way to think about it,

  • the molar mass of glucose, well then we just divide

  • the mass of our sample by the mass per mole

  • and we'll know how many moles we have.

  • So what is the molar mass of glucose?

  • Well to figure that out, and that's why

  • this periodic table of elements is useful,

  • we just have to figure out the molar mass

  • of the constituent elements.

  • So if we first look at carbon,

  • carbon, we see from this periodic table of elements,

  • has a molar mass of 12.01 grams per mole.

  • We've talked about it in other videos,

  • you could view this 12.01 as a relative atomic mass

  • of a carbon atom,

  • of as the average atomic mass of a carbon atom,

  • or what's useful, and this is where

  • Avogadro's Number is valuable,

  • if you have Avogadro's Number of carbons,

  • it is going to have a mass of 12.01 grams.

  • So carbon has a molar mass of 12.01 grams per mole

  • and now we can think about hydrogen in the same way.

  • Hydrogen has a molar mass of 1.008 grams per mole,

  • 008 grams per mole.

  • And then last but not least, we have oxygen here.

  • Oxygen, we can see from our periodic table of elements,

  • it has a molar mass of 16.00 grams per mole.

  • And so now we have all the information we need

  • from our periodic table of elements.

  • So the molar mass of glucose is going to be six

  • times the molar mass of carbon

  • plus 12 times the molar mass of hydrogen

  • plus six times the molar mass of oxygen.

  • So it's going to be six times 12.01 grams per mole

  • plus 12 times 1.008 grams per mole

  • plus every molecule of glucose has six oxygen

  • plus six times 16.00 grams per mole.

  • Six times 12.01

  • plus 12 times 1.008

  • plus six times 16

  • is equal to, and if we're thinking

  • about significant figures here,

  • the molar mass of hydrogen goes to the thousandths place

  • but we only go to the hundredths for carbon

  • and for oxygen, we're adding all of these up together

  • so it's going to be 180.

  • I can only go to the hundredths place

  • for significant figures, so 180.16.

  • So that's equal to 180.16 grams per mole.

  • And we could say grams of glucose, C6H12O6

  • per mole of glucose, C6H12O6

  • and then we can use this 1.52 kilograms

  • to figure out how many moles we have.

  • So if we start off with 1.52 kilograms of glucose,

  • so that's C6H12O6,

  • well first we can convert it to grams

  • 'cause here, our molar mass is given in terms of grams,

  • so times, we're going to want kilograms

  • in the denominator and grams in the numerator,

  • so how many grams are there per kilograms?

  • Well, we have 1,000 grams

  • for every one kilogram.

  • So when you multiply these two out,

  • this is going to give you the number of grams

  • we have of glucose which would be 1,520

  • and if you have your mass in terms of grams,

  • you can then divide by your molar mass

  • or you can view it as multiplying it by the moles per gram.

  • So for every one mole of glucose, C6H12O6,

  • we have 180.16 grams of glucose,

  • C6H12O6,

  • and this is going to get us,

  • we get 1.52 times 1,000 is equal to,

  • this is the number of grams of glucose we have,

  • and then we're going to divide by 180.16,

  • divide by 180.16,

  • gives us this number,

  • and let's see, if we see significant figures,

  • we have three significant figures here,

  • we have five here so we wanna round it

  • to three significant figures,

  • so it will be 8.44 moles of glucose.

  • So our kilograms cancel with our kilograms

  • and then our grams of glucose

  • cancel with our grams of glucose

  • and we are left with 8.44 moles of glucose,

  • moles of C6H12O6.

  • And we are done.

- [Instructor] We are asked to calculate

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工作實例。計算摩爾品質和摩爾數|AP化學|可汗學院 (Worked example: Calculating molar mass and number of moles | AP Chemistry | Khan Academy)

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    林宜悉 發佈於 2021 年 01 月 14 日
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