[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:03.89,Default,,0000,0000,0000,,I think it makes sense to keep doing a few more problems just
Dialogue: 0,0:00:03.90,0:00:05.78,Default,,0000,0000,0000,,so you really get intimately comfortable
Dialogue: 0,0:00:05.78,0:00:08.69,Default,,0000,0000,0000,,with PV equals nRT, or the ideal gas equation.
Dialogue: 0,0:00:08.70,0:00:16.11,Default,,0000,0000,0000,,So we have 98 milliliters of an unknown gas.
Dialogue: 0,0:00:16.12,0:00:20.98,Default,,0000,0000,0000,,They say weighs in the problem, but I'll use mass.
Dialogue: 0,0:00:20.99,0:00:24.31,Default,,0000,0000,0000,,Its mass is 0.081 grams.
Dialogue: 0,0:00:24.32,0:00:27.42,Default,,0000,0000,0000,,You shouldn't say weighs 0.081 grams. This is mass.
Dialogue: 0,0:00:27.43,0:00:29.69,Default,,0000,0000,0000,,Weight would be in newtons if we're in the metric system
Dialogue: 0,0:00:29.70,0:00:33.05,Default,,0000,0000,0000,,at standard temperature and pressure.
Dialogue: 0,0:00:33.06,0:00:37.19,Default,,0000,0000,0000,,Calculate the molar mass of the gas.
Dialogue: 0,0:00:37.20,0:00:40.13,Default,,0000,0000,0000,,So they want to know the mass per mole.
Dialogue: 0,0:00:40.14,0:00:50.09,Default,,0000,0000,0000,,So the molar mass, or the mass per mole.
Dialogue: 0,0:00:50.10,0:00:55.65,Default,,0000,0000,0000,,Now, we could just substitute this into PV is equal to nRT.
Dialogue: 0,0:00:55.66,0:00:57.98,Default,,0000,0000,0000,,We know what standard temperature and pressure are.
Dialogue: 0,0:00:57.99,0:01:00.81,Default,,0000,0000,0000,,Standard temperature is 273 degrees Kelvin.
Dialogue: 0,0:01:00.82,0:01:10.23,Default,,0000,0000,0000,,Standard pressure is 1 atmosphere.
Dialogue: 0,0:01:10.24,0:01:15.48,Default,,0000,0000,0000,,And then, of course, they're giving us 98 milliliters.
Dialogue: 0,0:01:15.49,0:01:20.84,Default,,0000,0000,0000,,And then you can just solve for how many moles we have.
Dialogue: 0,0:01:20.85,0:01:22.00,Default,,0000,0000,0000,,And naturally we'll do that.
Dialogue: 0,0:01:22.00,0:01:23.86,Default,,0000,0000,0000,,But the other way to think about it is that
Dialogue: 0,0:01:23.87,0:01:26.77,Default,,0000,0000,0000,,at standard temperature and pressure, an ideal gas
Dialogue: 0,0:01:26.78,0:01:29.74,Default,,0000,0000,0000,,-- and we did this two videos ago.
Dialogue: 0,0:01:29.75,0:01:35.75,Default,,0000,0000,0000,,We said 1 mole of an ideal gas at standard temperature and pressure
Dialogue: 0,0:01:35.76,0:01:39.46,Default,,0000,0000,0000,,will fill up a volume of 22.4 liters.
Dialogue: 0,0:01:39.46,0:01:42.70,Default,,0000,0000,0000,,And I'm normally not a big fan of memorizing things,
Dialogue: 0,0:01:42.71,0:01:44.80,Default,,0000,0000,0000,,but this might be something handy to memorize
Dialogue: 0,0:01:44.80,0:01:46.74,Default,,0000,0000,0000,,if you want to get through your chemistry test fast.
Dialogue: 0,0:01:46.75,0:01:48.60,Default,,0000,0000,0000,,But you can always derive this if you know
Dialogue: 0,0:01:48.61,0:01:51.83,Default,,0000,0000,0000,,what standard temperature and pressure is by PV equals nRT.
Dialogue: 0,0:01:51.83,0:01:55.67,Default,,0000,0000,0000,,But if you know that 1 mole is going to take up that much space,
Dialogue: 0,0:01:55.68,0:02:03.64,Default,,0000,0000,0000,,so one mole is to 22.4 liters, as how many moles,
Dialogue: 0,0:02:03.65,0:02:05.70,Default,,0000,0000,0000,,let's say x moles in this question.
Dialogue: 0,0:02:05.71,0:02:08.17,Default,,0000,0000,0000,,We want to figure out how many moles of the gas we have
Dialogue: 0,0:02:08.18,0:02:13.83,Default,,0000,0000,0000,,and we know we are at 98 milliliters, so 0.098,
Dialogue: 0,0:02:13.84,0:02:15.89,Default,,0000,0000,0000,,so we set up this proportional equation
Dialogue: 0,0:02:15.90,0:02:18.23,Default,,0000,0000,0000,,and we could figure out how many moles we're dealing with.
Dialogue: 0,0:02:18.24,0:02:21.34,Default,,0000,0000,0000,,If 1 mole takes up 22.4 liters,
Dialogue: 0,0:02:21.35,0:02:24.16,Default,,0000,0000,0000,,then our number of moles are going to take up 0.098,
Dialogue: 0,0:02:24.17,0:02:26.49,Default,,0000,0000,0000,,and this is an ideal gas in both circumstances.
Dialogue: 0,0:02:26.50,0:02:33.53,Default,,0000,0000,0000,,So we could say 22.4x is equal to 0.098.
Dialogue: 0,0:02:33.54,0:02:40.92,Default,,0000,0000,0000,,And then we have x is equal to 0.098 divided by 22.4.
Dialogue: 0,0:02:40.93,0:02:42.60,Default,,0000,0000,0000,,And this, of course, is in moles.
Dialogue: 0,0:02:42.61,0:03:04.99,Default,,0000,0000,0000,,So 0.098 divided by 22.4 is equal to 0.004375 moles.
Dialogue: 0,0:03:05.00,0:03:13.03,Default,,0000,0000,0000,,And they're telling us that this amount has a mass of 0.081 grams.
Dialogue: 0,0:03:13.04,0:03:14.94,Default,,0000,0000,0000,,So let me get the number.
Dialogue: 0,0:03:14.95,0:03:17.78,Default,,0000,0000,0000,,So how many grams are there per mole?
Dialogue: 0,0:03:17.79,0:03:23.95,Default,,0000,0000,0000,,So we take 0.081 grams and we just did the math
Dialogue: 0,0:03:23.96,0:03:30.92,Default,,0000,0000,0000,,to figure out that we're dealing with 0.004375 moles.
Dialogue: 0,0:03:30.93,0:03:33.56,Default,,0000,0000,0000,,So how many grams do we have per mole?
Dialogue: 0,0:03:33.56,0:03:36.01,Default,,0000,0000,0000,,Let's take the calculator out.
Dialogue: 0,0:03:36.02,0:03:49.08,Default,,0000,0000,0000,,So we have 0.081 divided by 0.004375 is equal to 18.51.
Dialogue: 0,0:03:49.10,0:04:01.88,Default,,0000,0000,0000,,So it's equals 18.5 grams per mole.
Dialogue: 0,0:04:01.88,0:04:03.40,Default,,0000,0000,0000,,So now this is an interesting question.
Dialogue: 0,0:04:03.41,0:04:06.82,Default,,0000,0000,0000,,So we figured out the molar mass of our mystery substance
Dialogue: 0,0:04:06.83,0:04:12.02,Default,,0000,0000,0000,,that took up 98 milliliters and had a mass of 0.081 grams
Dialogue: 0,0:04:12.03,0:04:14.39,Default,,0000,0000,0000,,at standard temperature and pressure, and we figured out
Dialogue: 0,0:04:14.40,0:04:18.67,Default,,0000,0000,0000,,its molar mass, or its mass per mole, is 18.5 grams.
Dialogue: 0,0:04:18.68,0:04:23.87,Default,,0000,0000,0000,,So any guess as to what molecule we're dealing with?
Dialogue: 0,0:04:25.70,0:04:27.99,Default,,0000,0000,0000,,And probably it's not going to be exact,
Dialogue: 0,0:04:28.00,0:04:31.66,Default,,0000,0000,0000,,but at least in my brain, water seems to be a good candidate.
Dialogue: 0,0:04:31.67,0:04:33.93,Default,,0000,0000,0000,,Water is H20.
Dialogue: 0,0:04:33.94,0:04:37.83,Default,,0000,0000,0000,,Maybe I should do it in blue because it's water. H2O.
Dialogue: 0,0:04:37.84,0:04:40.40,Default,,0000,0000,0000,,The mass of each hydrogen is 1.
Dialogue: 0,0:04:40.41,0:04:43.88,Default,,0000,0000,0000,,Remember, hydrogen, at least in its most standard form,
Dialogue: 0,0:04:43.89,0:04:45.92,Default,,0000,0000,0000,,doesn't have a neutron, so it's really just a proton
Dialogue: 0,0:04:45.92,0:04:48.27,Default,,0000,0000,0000,,and an electron, so it has an atomic mass of 1
Dialogue: 0,0:04:48.28,0:04:50.19,Default,,0000,0000,0000,,or a molar mass of 1 gram.
Dialogue: 0,0:04:50.20,0:04:52.97,Default,,0000,0000,0000,,And oxygen has a molar mass of 16 grams.
Dialogue: 0,0:04:52.98,0:04:57.41,Default,,0000,0000,0000,,So you have two hydrogens, so it's 2 plus 16 is equal to 18.
Dialogue: 0,0:04:57.42,0:05:01.54,Default,,0000,0000,0000,,So it looks like our mystery substance is water.