Name: Worked example: Calculating the mass of a substance in a mixture | AP Chemistry | Khan Academy
Uploaded: 2020-04-06T07:50:02.000Z
Duration: 5 min 21 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...

- [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

How many grams of potassium chloride are in

So pause this video and see if you can figure that

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

but you could also view it as 39.10 grams per mole

And so, how do we think about how many grams

of potassium chloride are in the potassium supplement?

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

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

So that's how many grams 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,

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

well then I know the mass of potassium chloride,

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

has a mass of how many grams of potassium?

So this calculation will tell me moles of potassium.

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

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

And so, this calculation is going to tell me

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,

times 74.55, is going to be equal to that.

And then let me think about my significant figures

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

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

Subtitles ListPlay Video

Worked example: Calculating the mass of a substance in a mixture | AP Chemistry | Khan Academy

equivalent

significant

bunch

figure