Converting Between Grams and Moles (Part 2)

This video is about

converting

between

grams

and

moles

,

part

2. I'm going to do a couple more practice problems on this topic. If you haven't seen the first video yet, you might want to watch it first and then come back and watch it. What is the mass in

grams

of 0.850

moles

of sulfur dioxide (SO2)? Okay, moles and grams. The first thing you should think about is what is the molar mass of SO2? I have sulfur and oxygen here, so I look them up on the periodic table. Here are their molar masses and now I add them. I have a sulfur here, so I make 1 times this number but I have two oxygen, I have this O2 here, so I make 2 times the molar mass of oxygen.

If we add them up, we get this, which tells me that the molar mass of sulfur dioxide (SO2) is 64.07 g/mol, which means that 1 mole of SO2 weighs 64.07 grams. So now we can think about it, okay? If we had 1 mole of SO2, it would weigh 64.7 grams. If I had 2 moles of SO2, I would multiply this number by 2 (this number is how much 1 mole would weigh) and I get this, okay? I don't have 1 mole, I don't have 2 moles, I have 0.850 moles which is actually less than 1 mole. Okay, so in grams, I'm going to get a number less than this because I have less than 1 mole.

But the calculations I'm going to do are exactly the same, okay? I know how much a mole weighs, so I'll take this amount and multiply it by the number of moles I have, which is 0.850. Do these calculations and I get 54.5 grams which, as we predicted, is less than the amount that one mole weighs. Although this number is less than 1 mole, the calculations we are doing now are exactly the same, okay? We're going from moles to grams, so we multiply the moles by the molar mass and this is what I get. Now for the conversion factor: 0.850 moles of SO2, multiply it by the conversion factor made from the molar mass information.

I'm going to use this one here because it puts moles of SO2 at the bottom, there's one at the top here, cancel and cancel, hold this, keep doing the math, this multiplies this divided by this and I got what I got. up here 54.5 grams of SO2. And the math is exactly the same for this because dividing by one here doesn't change the value. All I do is multiply moles by molar mass to go from moles to grams. One last question! How many moles are there in 32.7 grams of ethanol (C2H6O)? Well, first question, how much does a mole of ethanol weigh?

What is the molar mass? It's made of carbon, hydrogen, and oxygen, so I look up all three on the periodic table and add their molar masses depending on how many of each I have, okay? I have C2, so I have 2 carbons, 2 times this. I have 6 hydrogens, so I do 6 times this and I have only 1 oxygen, so I do only 1 time this and I get this as my number, 46.08, which tells us that the molar mass of C2H6O is 46.08 g/mol . which means that 1 mole of C2H6O, Ethanol, weighs 46.08 grams. I have a certain number of grams, I want to know how many moles they are, okay?

Well, if I had 46.08 grams, I know I would have 1 mole. If I had 92.16 grams for example, well that's more than this, I want to know how many times this number goes into this? How many times could this fit into this? So to solve it, you would divide by this number and you would get 2 moles, and it turns out that it is exactly twice as much. Now the number of grams I have is 32.7 g, which is actually less than the number of grams in just one mole. So the number I'll end up with will be less than one mole, but the math is exactly the same.

How many times does this number enter this number? That's what I want to know, so I'm going to take this number and divide it by the molar mass. I want to know how many of these fit in this. It's going to be less than 1, I do this division and I get 0.710 moles, right? Less than 1 mole because this number here is less than what 1 mole weighs. Although I get this answer that it is less than one and this is less than the amount that a mole weighs, the calculations I do are always the same when I go from grams to moles.

I divide the grams by the molar mass, which is what I'm doing in all of these examples here. And finally, for the conversion factor, 32.7 g of C2H6O multiplied by the conversion factor based on this molar mass information. I want grams of C2H6O at the bottom, so 1 mole of C2H6O will be at the top, this cancels out, this cancels out, leaving me with moles of ethanol here. And the math is this multiplied by 1 divided by this and you get 0.710 moles of C2H6O. And I'll point out again that the math for the conversion factor is exactly the same as the math for the think it through method because multiplying 32.7 by 1 doesn't change it, so all we're really doing is 32.7 divided for 46.08.

This is how we convert between moles and grams.