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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,

B2 中上級