字幕表 動画を再生する 英語字幕をプリント - [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.