WEBVTT 00:00:00.000 --> 00:00:03.050 Let's talk about a concept that probably 00:00:03.070 --> 00:00:06.980 confuses chemistry students the most on some level. 00:00:06.990 --> 00:00:09.280 But on some level it's also one of the simplest concepts. 00:00:09.300 --> 00:00:10.870 And that's the idea of a mole, 00:00:12.050 --> 00:00:13.590 which in chemistry is different 00:00:13.650 --> 00:00:15.370 that the thing digging up your backyard, 00:00:15.510 --> 00:00:19.140 or the thing you want to get removed from your left eye. 00:00:19.270 --> 00:00:21.800 A mole in chemistry is just a number. 00:00:21.820 --> 00:00:23.770 It's just a number, 00:00:23.790 --> 00:00:28.200 and the number is 6.02 times 10 to the 23. 00:00:28.220 --> 00:00:29.730 So it's a very huge number. 00:00:29.740 --> 00:00:31.750 And this is also called Avogadro's number. 00:00:31.770 --> 00:00:34.560 Maybe I will do a video on Avogadro. 00:00:35.020 --> 00:00:38.630 Avogadro's number 00:00:39.790 --> 00:00:41.920 But that's all you need to know. 00:00:41.940 --> 00:00:43.460 A mole is just a number. 00:00:43.480 --> 00:00:47.310 There are kind of more Byzantine definitions of a mole. 00:00:47.320 --> 00:00:49.200 This actually is not 00:00:49.210 --> 00:00:51.590 -- actually, let me copy and paste it from Wikipedia. 00:00:51.610 --> 00:00:55.430 This is Wikipedia's definition of a mole. 00:00:55.450 --> 00:00:58.430 And you hopefully at the end of this video 00:00:58.440 --> 00:00:59.740 you'll see that they're equivalent. 00:00:59.740 --> 00:01:01.420 But if you're just getting exposed to the concept, 00:01:01.430 --> 00:01:07.140 this to me, it's just not an easy concept. 00:01:07.160 --> 00:01:10.400 Basically, "a mole is defined as the amount of substance 00:01:10.420 --> 00:01:16.060 of a system that contains as many elemental entities 00:01:16.070 --> 00:01:19.860 as there are atoms in 12 grams of carbon 12." 00:01:19.880 --> 00:01:24.490 Well, I just told you that a mole is 6.02 times 10 to the 23. 00:01:24.510 --> 00:01:26.460 So if you just take the last part, 00:01:26.480 --> 00:01:30.850 atoms in 12 grams of carbon 12. 00:01:30.870 --> 00:01:39.930 So that means that there are 1 mole of carbon 12 00:01:39.950 --> 00:01:42.380 -- let me write it like that -- carbon 12. 00:01:42.400 --> 00:01:57.730 There are 1 mole of carbon 12 atoms in 12 grams of carbon. 00:01:57.750 --> 00:01:59.930 And so that's why a mole is useful. 00:01:59.950 --> 00:02:02.260 So I could have just instead of writing 1 mole, 00:02:02.280 --> 00:02:06.350 I could have replaced this as there's 6.02 times 10 00:02:06.360 --> 00:02:12.080 to the 23 carbon atoms, carbon 12 atoms in 12 grams of carbon. 00:02:12.090 --> 00:02:13.250 How do you figure that out? 00:02:13.260 --> 00:02:15.430 Or I guess, what else does this mean? 00:02:15.450 --> 00:02:17.370 I mean, we just added in carbon, 00:02:17.390 --> 00:02:19.570 they said it's the amount of substance of any molecule, 00:02:19.580 --> 00:02:22.960 if you convert between atomic mass units and grams. 00:02:22.980 --> 00:02:24.610 This I find very confusing. 00:02:24.620 --> 00:02:26.620 How can we apply this in other places? 00:02:26.640 --> 00:02:29.010 So the first thing to realize is a mole is just a way 00:02:29.030 --> 00:02:32.390 of translating between grams and atomic mass units. 00:02:32.410 --> 00:02:35.340 One carbon 12 atom is what? 00:02:35.360 --> 00:02:37.720 What's its mass number? 00:02:37.740 --> 00:02:38.990 It's 12. 00:02:39.010 --> 00:02:41.700 That's why it's called carbon 12 instead of carbon 14. 00:02:41.710 --> 00:02:46.460 So its mass is 12 atomic mass units. 00:02:46.470 --> 00:02:50.780 So if you have something that has a mass of 12 atomic mass units 00:02:50.800 --> 00:02:52.210 and you have a mole of them, 00:02:52.490 --> 00:02:55.640 or you have 6.02 times 10 to the 23 of them, 00:02:55.660 --> 00:03:01.530 all of those atoms combined will have a mass of 12 grams. 00:03:01.550 --> 00:03:03.810 So another way to think about it is 00:03:04.070 --> 00:03:13.740 1 gram is equal to 1 mole of atomic mass units. 00:03:13.760 --> 00:03:16.620 I'll write amu's like that. 00:03:16.640 --> 00:03:22.160 Or you can write 1 gram is equal to 6.02 00:03:22.170 --> 00:03:26.200 times 10 to the 23 atomic mass units. 00:03:26.220 --> 00:03:28.560 And the reason why this is useful 00:03:28.570 --> 00:03:30.700 -- and it's kind of addressed in this Wikipedia definition there 00:03:30.710 --> 00:03:33.800 -- is it helps us translate between the atomic world 00:03:33.820 --> 00:03:36.350 -- where we deal with atomic mass units and we deal with, 00:03:36.360 --> 00:03:37.970 oh, we've got an extra neutron now, 00:03:37.980 --> 00:03:41.130 let's add one to our atomic mass number 00:03:41.150 --> 00:03:43.070 -- and translating between that atomic world 00:03:43.120 --> 00:03:45.460 and our everyday world where we deal in grams. 00:03:45.470 --> 00:03:47.120 And just so you know, a gram is still 00:03:47.140 --> 00:03:50.710 a pretty small amount of mass. 00:03:50.720 --> 00:03:52.460 It's 1/1,000 of a kilogram. 00:03:52.480 --> 00:03:54.070 A kilogram is about 2 pounds. 00:03:54.090 --> 00:03:56.500 So this is about 1/500 of a pound. 00:03:56.510 --> 00:03:58.130 So this is not much. 00:03:58.150 --> 00:04:02.010 So there's a ton of atoms in a very small amount of 00:04:02.030 --> 00:04:06.580 -- in 1 gram of carbon, or at least in 12 grams of carbon, 00:04:06.600 --> 00:04:10.790 you have a ton of atoms. You have 6.02 times 10 to the 23. 00:04:10.810 --> 00:04:12.160 And just to hit the point home, 00:04:12.170 --> 00:04:13.800 I probably should have talked about this in the atom. 00:04:13.820 --> 00:04:15.970 This is a huge number. 00:04:15.980 --> 00:04:22.110 To maybe visualize it, if you think of -- I was told that 00:04:22.130 --> 00:04:25.290 in the diameter of a hair, if this is a hair 00:04:25.300 --> 00:04:27.640 and this is diameter of the hair, 00:04:27.660 --> 00:04:32.890 if you go this way there 1 million carbon atoms. 00:04:32.910 --> 00:04:35.470 1 million carbon atoms that way. 00:04:35.480 --> 00:04:41.550 Or if you were to take an apple 00:04:41.560 --> 00:04:45.180 and you were to try to figure out what fraction, 00:04:45.200 --> 00:04:48.780 if you were to make one of the atoms of an apple 00:04:48.800 --> 00:04:50.140 -- and obviously, 00:04:50.150 --> 00:04:52.750 an apple has a bunch of different types of atoms in it 00:04:52.760 --> 00:04:53.830 -- but if you were to take one of the atoms 00:04:53.850 --> 00:04:55.510 and make it the size of the apple, 00:04:55.530 --> 00:04:58.300 then the apple would be the size of the earth. 00:04:58.320 --> 00:05:00.760 So an apple atom is to an apple 00:05:00.910 --> 00:05:03.000 as an apple is to the earth. 00:05:03.020 --> 00:05:06.310 So these are obviously-- it's hard for us 00:05:06.330 --> 00:05:08.780 to even process things of this size. 00:05:08.800 --> 00:05:12.130 When you just have one gram of 00:05:12.150 --> 00:05:14.770 -- well, let's say you have 1 gram of hydrogen. 00:05:14.780 --> 00:05:22.920 1 gram of hydrogen. 1 gram of hydrogen. 00:05:22.930 --> 00:05:24.590 If you have 1 gram of hydrogen, 00:05:24.610 --> 00:05:29.500 that means you have 1 mole of hydrogen. 00:05:29.510 --> 00:05:31.150 How do I know that? 00:05:31.170 --> 00:05:34.400 Because hydrogen's atomic mass number is 1. 00:05:34.420 --> 00:05:39.090 So in general, if you just take any element 00:05:39.120 --> 00:05:41.690 -- so what is the mass of, 00:05:41.720 --> 00:05:45.160 let me just pick, 1 mole of aluminum? 00:05:45.170 --> 00:05:54.170 So if I were to take 6.02 times 10 to the 23 aluminum atoms, 00:05:54.180 --> 00:05:56.170 what is the mass of that collection? 00:05:56.180 --> 00:06:00.720 Well, each of them have an atomic mass number of 13. 00:06:00.740 --> 00:06:08.070 So it's 13 amu's-- I don't have to put the s there 00:06:08.090 --> 00:06:11.740 -- times six point 00:06:11.750 --> 00:06:14.360 -- well, I won't write that way, actually. 00:06:14.380 --> 00:06:15.700 That'll probably just confuse you. 00:06:15.710 --> 00:06:18.050 The easy way to think about is if you have a mole of an atom, 00:06:18.080 --> 00:06:21.100 you take its mass-- I was taking its atomic number, 00:06:21.120 --> 00:06:23.930 that's not good-- you take its mass number. 00:06:23.950 --> 00:06:26.070 In this case let's say it's 27. 00:06:26.090 --> 00:06:30.370 So we're dealing with aluminum 27. 00:06:30.380 --> 00:06:34.970 You take its mass number, and if you have 1 mole of it, 00:06:34.980 --> 00:06:41.690 then the mass of that will be 27 grams. 00:06:41.710 --> 00:06:44.420 So that literally, when you have one mole of an atom 00:06:44.440 --> 00:06:48.470 it's a direct translation between its mass number and grams. 00:06:48.480 --> 00:06:53.800 1 mole of iron, let's say iron 56-- 00:06:53.810 --> 00:06:55.770 there's obviously many isotopes or iron 00:06:55.780 --> 00:06:58.310 -- let's say we're dealing with iron 56. 00:06:58.330 --> 00:06:59.850 You normally don't hear it like that, 00:06:59.860 --> 00:07:02.480 but let's say we're dealing with the isotope of iron 00:07:02.500 --> 00:07:04.180 that has a mass number of 56. 00:07:04.200 --> 00:07:11.410 So if I have 1 mole of this, 1 mole of this atom right here, 00:07:11.420 --> 00:07:14.380 that's going to have a mass of 00:07:15.200 --> 00:07:16.620 -- the math isn't difficult here 00:07:16.640 --> 00:07:23.050 -- 56 grams. Right? And if you think about it, 00:07:23.060 --> 00:07:24.950 how many atomic mass units is this? 00:07:24.970 --> 00:07:30.930 Well, this is 56 atomic mass units per atom. 00:07:30.950 --> 00:07:32.600 Then you have a mole of those, so you have 00:07:32.620 --> 00:07:37.830 6.02 times 10 to the 23 times 56 atomic mass units. 00:07:37.840 --> 00:07:38.980 And then you divide it 00:07:39.000 --> 00:07:41.750 by the number of atomic mass units per gram. 00:07:41.770 --> 00:07:43.460 And you end up with 56 grams. 00:07:43.470 --> 00:07:44.840 But the easy way to think about it is 00:07:44.860 --> 00:07:46.920 you just take whatever the mass number is. 00:07:46.930 --> 00:07:49.690 If you have silicon, if you have a mole of silicon, 00:07:49.700 --> 00:07:52.200 a mole of silicon will have a mass 00:07:52.400 --> 00:07:53.240 -- I don't want to say weight 00:07:53.310 --> 00:07:54.840 because this should apply to any planet 00:07:54.970 --> 00:07:56.320 -- of 28 grams. 00:07:56.360 --> 00:08:04.220 What about 2 moles of silicon? 2 moles of silicon. 00:08:04.250 --> 00:08:05.360 And I'll write its mass number. 00:08:05.380 --> 00:08:10.500 Let's say silicon has a mass number of 28. 00:08:10.510 --> 00:08:12.890 Two moles of silicon. 00:08:12.910 --> 00:08:15.770 Well, 1 more would have a mass of 28 grams, 00:08:15.790 --> 00:08:23.890 so 2 moles is going to have a mass of 56 grams. 00:08:23.910 --> 00:08:29.960 If I were to say, let's say I had 4 moles of oxygen, 00:08:29.960 --> 00:08:32.390 which has a mass number of 16. 00:08:32.410 --> 00:08:34.600 What is the mass of that? 00:08:34.610 --> 00:08:37.320 This is a huge number of oxygen atoms 00:08:37.330 --> 00:08:38.760 -- what would be the mass of that? 00:08:38.780 --> 00:08:44.100 Well, it would be 4 times-- 1 mole of oxygen 00:08:44.120 --> 00:08:46.950 would have a mass of 16 grams, 00:08:47.060 --> 00:08:49.870 so 4 moles has 64 grams. 00:08:52.320 --> 00:08:55.400 It's confusing because we're not used to using a word 00:08:55.420 --> 00:08:58.360 like moles as a number, but all it is, is a number. 00:08:58.380 --> 00:09:00.350 And the easy way to think about is that it lets us 00:09:00.360 --> 00:09:06.420 translate between this atomic mass unit number and grams. 00:09:06.430 --> 00:09:08.440 And you say, well, how do I get that many grams? 00:09:08.460 --> 00:09:12.020 Well, I have to have 6.02 times 10 to the 23 carbon atoms 00:09:12.030 --> 00:09:16.300 for that collection of carbon to have a mass of 12 grams. 00:09:16.320 --> 00:09:18.470 That's all that mole means. 00:09:18.480 --> 00:09:19.460 It's just a number. 00:09:19.640 --> 00:09:21.360 And I encourage you to kind of play around 00:09:21.450 --> 00:09:22.800 with a lot of what we talked about. 00:09:22.800 --> 00:09:24.210 Because it's super important 00:09:24.270 --> 00:09:25.140 to have the intuition behind moles, 00:09:25.140 --> 00:09:27.780 otherwise, you'll get confused later on 00:09:27.780 --> 00:09:29.790 when we start getting into energies 00:09:29.860 --> 00:09:31.470 in terms of it requires kilojoules per mole, 00:09:31.470 --> 00:09:34.330 and what is the energy of this reaction 00:09:34.330 --> 00:09:35.310 and all that type of stuff. 00:09:35.310 --> 00:09:37.970 So just really try to make sure 00:09:37.970 --> 00:09:38.530 you digest this as well as possible. 00:09:38.530 --> 00:09:40.290 And let me know if you don't 00:09:40.290 --> 00:09:42.590 and I'll maybe make another video on this 00:09:42.590 --> 00:09:43.060 because it's so important.