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  • - [Instructor] We're told that a 0.450 gram

  • potassium supplement contains 22% potassium by mass.

  • The potassium is present in the supplement

  • as potassium chloride, which has a molar mass

  • of 74.55 grams per mole.

  • How many grams of potassium chloride are in

  • the potassium supplement?

  • So pause this video and see if you can figure that

  • on your own, and I will give you a hint.

  • You might need a periodic table,

  • so I'll give you one right over there.

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

  • So they already give us the molar mass

  • of potassium chloride,

  • but it'll be useful for us to know

  • the molar mass of potassium itself.

  • And we can see it's 39.10.

  • So for potassium, it is 39.10.

  • That's its average atomic mass,

  • but you could also view it as 39.10 grams per mole

  • of potassium, could write it like that.

  • And that is, I think, all we will need

  • the periodic table of elements for.

  • And so, how do we think about how many grams

  • of potassium chloride are in the potassium supplement?

  • Well what I could do is, I could say,

  • "All right, what's the total mass of my supplement,"

  • and multiply it by 22% to figure out

  • what is the mass of potassium I have.

  • So let me figure that out.

  • So mass of potassium,

  • that is going to be my 0.450 grams

  • times 22%, which is going to be equal to,

  • I'll get my calculator out,

  • 0.450 times 22%, or .22,

  • is equal to 0.099.

  • 0.099.

  • So that's how many grams of potassium I have.

  • And now I can use that to figure out

  • how many moles of potassium I have.

  • And if I know how many moles of potassium I have,

  • all the potassium is present as potassium chloride.

  • Because then I'll have the same number of moles

  • of potassium chloride, and then I could figure out,

  • well, based on.

  • And if I know the number of moles of potassium chloride,

  • well then I know the mass of potassium chloride,

  • because they give us the molar mass.

  • So let's do that.

  • So we have 0.099

  • grams of potassium.

  • And I wanna figure out how many moles of potassium I have.

  • So I'm gonna multiply that times,

  • one mole of potassium

  • has a mass of how many grams of potassium?

  • And we can see it right over there.

  • It's 39.10 grams per mole.

  • So 39.10.

  • One mole is equivalent to 39.10 grams,

  • if we're dealing with potassium.

  • So this calculation will tell me moles of potassium.

  • Now, I have exactly one potassium atom

  • for every potassium chloride molecule.

  • And so, whatever this number is,

  • I'm gonna have the same number of moles

  • of potassium chloride,

  • but just to make that clear, I will write this down.

  • I have one mole

  • of potassium chloride for every one

  • mole of potassium.

  • And you could already see how the units will cancel out.

  • That will cancel with that.

  • That will cancel with that.

  • And so, this calculation is going to tell me

  • moles of potassium chloride,

  • but I wanna know the mass of potassium chloride.

  • So then I will multiply this by the molar mass,

  • times 74, and I'm gonna do this in another color,

  • just makes it look nice, 74.55 grams

  • of potassium chloride

  • for every mole

  • of potassium chloride.

  • And so then, this will cancel with this,

  • and this will tell me how many grams

  • of potassium chloride I actually have.

  • So let's get the calculator out again.

  • So I have 0.099

  • divided by 39.10,

  • divided by 39.10,

  • times one, times 74.55,

  • times 74.55, is going to be equal to that.

  • And then let me think about my significant figures

  • or my significant digits here.

  • So let's see, out of all of the information I used,

  • I have three significant figures here,

  • I have three here, I have four here,

  • I have four here.

  • So I'm multiplying a bunch of things and dividing,

  • so I have to take the minimum number

  • of significant figures, which is three.

  • So I will round to three significant figures,

  • which is 0.189 grams.

  • So this is,

  • let me do this in a new color,

  • 0.189 grams of potassium chloride,

  • which is exactly what they are asking us.

  • And we are done.

- [Instructor] We're told that a 0.450 gram

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B2 中上級

実践例。混合物中の物質の質量を計算する|AP化学|カーンアカデミー (Worked example: Calculating the mass of a substance in a mixture | AP Chemistry | Khan Academy)

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    林宜悉 に公開 2021 年 01 月 14 日
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