Converting Between Grams and Moles (Part 2)

This video is converting between grams and moles part 2. I'm going to do a couple more practice problems on this stuff. If you haven't yet seen the first video you might want to go check that out first and then come back and watch this. What is the mass in grams of 0.850 moles of sulfur dioxide (SO2)? Okay, moles and grams. The first thing you should think of is, what is the molar mass of SO2? Okay. I've got Sulfur and I've got Oxygen in here so I look them both up on the periodic table. Here are their molar masses. And now I add these together. I have one Sulfur here so I do 1 times this number but I've got two Oxygens, I've got this O2 here, two oxygens so I do 2 times the molar mass of Oxygen. Add these together and I 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 through it, okay? If we had 1 mole of SO2, it would weigh 64.07 grams. If I had 2 moles of SO2, I do 2 times this number (which 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 be getting a number that's smaller than this because I have less than 1 mole. But the math I'm going to be doing is exactly the same, okay? I know how much one mole weighs so I'm going to take this amount and I'm just going to multiply it by the number of moles that I have which is 0.850. Do this math and I get 54.5 grams which as we predicted is less than the amount that one mole weighs. Even though this number is less than 1 mole, the math that we're doing now is 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 this molar mass information. I'm going to use this one here because it puts moles SO2 on the bottom, there on the top here, cancel, cancel, keep this, go through doing the math, this times this divided by this and I got, as I got up here, 54.5 grams SO2. And the math is exactly the same for this as it was for this because dividing by one here doesn't change the value. All I'm doing is multiplying moles by molar mass to go from moles to grams. One more last question! How many moles are in 32.7 grams of ethanol (C2H6O)? Okay, first question, how much does one mole of Ethanol weigh? What's the molar mass? It's made of Carbon, Hydrogen and Oxygen so I look all three of them up on the periodic table and I add their molar masses together depending on how many of each I have, okay? I've got C2 so I've got 2 Carbons, 2 times this. I've got 6 Hydrogens so I do 6 times this and I've got just 1 Oxygen so I do just 1 times 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 that is, okay? Well if I had 46.08 grams, I know that I'd have 1 mole. If I had 92.16 grams for example, well that's more than this, I want to know how many times does this number go into this? How many times could this fit into this? So to figure that out, I would divide by this number and I get 2 moles, it turns out this is exactly double. Now the number of grams that I have is 32.7g which is actually less than the number of grams that is in just one mole. So the number that I'm going to end up with is going to be less than one mole but the math is exactly the same. How many times does this number go into this number? That's what I want to find out. So I'm going to take this number and I'm going to divide it by the molar mass, by this, I want to find out how many of these fit into 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 how much 1 mole weighs. Even though I get this answer that's less than one and this was less than the amount that one mole weighs, the math I'm doing is always the same when I go from grams to moles. I divide by grams by the molar mass which is what I'm doing in all these examples right here. And finally for the conversion factor, 32.7g C2H6O multiplied by conversion factor based on this molar mass information. I want grams of C2H6O on the bottom so 1 mole of C2H6O is going to be on the top, this cancels out, this cancels out, leaving me with moles of Ethanol here. And the math is this times 1 divided by this and you get 0.710 moles C2H6O. And I will point out once again that the math or the conversion factor is exactly the same as the math for the thinking it through method because multiplying 32.7 by 1 doesn't change it so all we're really doing is 32.7 divided by 46.08. So that is how we convert back and forth between moles and grams.