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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

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