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  • - [Instructor] So we have four different compounds here,

  • and we also have their molar masses

  • and what I would like you to do

  • is think about which of these compounds has the highest,

  • highest percentage of sulfur by mass.

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

  • All right, now one way to approach this

  • is to say for each of these compounds,

  • so if we say per mole

  • of each compound,

  • of each compound,

  • we could figure out how many grams, I would say

  • what is the mass,

  • what is the mass of sulfur

  • divided by the total mass of the compound?

  • Total, total mass of the compound.

  • Now the mass of sulfur,

  • we can see that for a mole of each of these compounds,

  • you have exactly one mole of sulfur.

  • In all of these formulas,

  • you see exactly one sulfur.

  • So in all of these situations,

  • the mass of sulfur is going to be the same.

  • This is going to be the same,

  • same for all compounds.

  • It wouldn't be the same for all compounds

  • if some of them had two sulfurs

  • or some of them had three

  • or maybe some of them had none,

  • but all of them have one sulfur,

  • so for a mole of each of these compounds,

  • you're going to have a mole of sulfur,

  • and so they're going to have the same mass of sulfur.

  • We could take out a periodic table

  • and we could look it up and we could say, oh, look.

  • The average atomic mass of sulfur is 32.06

  • and so if I had a mole of it,

  • it would be 32.06 grams,

  • so we could put the 32.06 grams up here, if we like.

  • This would be 32.06 grams

  • in all of these cases 'cause once again,

  • if you have a mole of each of these compounds,

  • you're going to have a mole of sulfur

  • because you only see one sulfur in these,

  • in each of these formulas,

  • but the big picture is in all of the cases,

  • I'm gonna have the same value in the numerator,

  • so I want to have the highest percentage,

  • I would want to minimize the value in the denominator.

  • So I would really want to see which one

  • would have the lowest denominator,

  • or which one would have the lowest mass

  • for a mole of the molecules

  • or another way of thinking about it,

  • which of them will have the lowest molar mass?

  • And you could see very clearly

  • that this compound right over here

  • has the lower, lowest molar mass.

  • So we're done.

  • This has the highest percentage of sulfur by mass

  • because per mole, it has the same mass of sulfur

  • as everything else,

  • but it has the lowest total mass,

  • so it's gonna have the highest percentage of sulfur.

  • So a quick way to do it,

  • we just looked at the lowest molar mass.

  • You could have gone and taken 32.06

  • and divided by each of these values

  • and figured out a percentage.

  • Well, they're not asking us to figure out the percentage.

  • They're saying which one has the highest percentage?

  • And that would be this second compound right over there.

- [Instructor] So we have four different compounds here,

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B2 中上級

コンマウンズ内の元素の質量%を比較する例 (Example comparing mass percentages of element in comounds)

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    林宜悉 に公開 2021 年 01 月 14 日
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