WEBVTT

00:00:00.000 --> 00:00:01.220


00:00:01.220 --> 00:00:03.480
I think it makes sense to keep
doing a few more problems just

00:00:03.480 --> 00:00:06.620
so you really get intimately
comfortable with PV equals

00:00:06.620 --> 00:00:08.950
nRT, or the ideal
gas equation.

00:00:08.950 --> 00:00:15.240
So we have 98 milliliters
of an unknown gas.

00:00:15.240 --> 00:00:17.980


00:00:17.980 --> 00:00:20.750
They say weighs in the problem,
but I'll use mass.

00:00:20.750 --> 00:00:26.130
Its mass is 0.081 grams. You
shouldn't say weighs 0.081

00:00:26.130 --> 00:00:27.170
grams. This is mass.

00:00:27.170 --> 00:00:29.950
Weight would be in newtons if
we're in the metric system at

00:00:29.950 --> 00:00:32.870
standard temperature
and pressure.

00:00:32.870 --> 00:00:37.740
Calculate the molar
mass of the gas.

00:00:37.740 --> 00:00:39.670
So they want to know
the mass per mole.

00:00:39.670 --> 00:00:49.750
So the molar mass, or
the mass per mole.

00:00:49.750 --> 00:00:55.540
Now, we could just substitute
this into PV is equal to nRT.

00:00:55.540 --> 00:00:57.760
We know what standard
temperature and pressure are.

00:00:57.760 --> 00:01:01.220
Standard temperature is
273 degrees Kelvin.

00:01:01.220 --> 00:01:06.410
Standard pressure
is 1 atmosphere.

00:01:06.410 --> 00:01:09.940


00:01:09.940 --> 00:01:15.220
And then, of course, they're
giving us 98 milliliters.

00:01:15.220 --> 00:01:20.295
And then you can just solve for
how many moles we have.

00:01:20.295 --> 00:01:21.730
And actually, maybe
I'll do that.

00:01:21.730 --> 00:01:24.210
But the other way to think about
it is that at standard

00:01:24.210 --> 00:01:27.010
temperature and pressure, an
ideal gas-- and we did this

00:01:27.010 --> 00:01:27.900
two videos ago.

00:01:27.900 --> 00:01:34.600
We said 1 mole of an ideal gas
at standard temperature and

00:01:34.600 --> 00:01:39.770
pressure will fill up a
volume of 22.4 liters.

00:01:39.770 --> 00:01:42.750
And I'm normally not a big fan
of memorizing things, but this

00:01:42.750 --> 00:01:45.160
might be something handy to
memorize if you want to get

00:01:45.160 --> 00:01:47.400
through your chemistry test
fast. But you can always

00:01:47.400 --> 00:01:49.180
derive this if you know what
standard temperature and

00:01:49.180 --> 00:01:51.720
pressure is by PV equals nRT.

00:01:51.720 --> 00:01:54.390
But if you know that 1 mole is
going to take up that much

00:01:54.390 --> 00:02:03.650
space, so one mole is to 22.4
liters, as how many moles,

00:02:03.650 --> 00:02:05.430
let's say x moles in
this question.

00:02:05.430 --> 00:02:08.490
We want to figure out how many
moles of the gas we have and

00:02:08.490 --> 00:02:14.140
we know we are at 98
milliliters, so 0.098, so we

00:02:14.140 --> 00:02:16.270
set up this proportional
equation and we could figure

00:02:16.270 --> 00:02:18.060
out how many moles we're
dealing with.

00:02:18.060 --> 00:02:22.240
If 1 mole takes up 22.4 liters,
then our number of

00:02:22.240 --> 00:02:25.030
moles are going to take up
0.098, and this is an ideal

00:02:25.030 --> 00:02:26.900
gas in both circumstances.

00:02:26.900 --> 00:02:34.250
So we could say 22.4x
is equal to 0.098.

00:02:34.250 --> 00:02:40.630
And then we have x is equal
to 0.098 divided by 22.4.

00:02:40.630 --> 00:02:43.310
And this, of course,
is in moles.

00:02:43.310 --> 00:03:05.480
So 0.098 divided by 22.4 is
equal to 0.004375 moles.

00:03:05.480 --> 00:03:10.540
And they're telling us that
this amount has a mass of

00:03:10.540 --> 00:03:15.520
0.081 grams. So let
me get the number.

00:03:15.520 --> 00:03:18.030
So how many grams are
there per mole?

00:03:18.030 --> 00:03:24.390
So we take 0.081 grams and we
just did the math to figure

00:03:24.390 --> 00:03:30.950
out that we're dealing
with 0.004375 moles.

00:03:30.950 --> 00:03:34.140
So how many grams do
we have per mole?

00:03:34.140 --> 00:03:35.610
Let's take the calculator out.

00:03:35.610 --> 00:03:50.530
So we have 0.081 divided by
0.004375 is equal to 18.51.

00:03:50.530 --> 00:03:58.840
So it's equals 18.5
grams per mole.

00:03:58.840 --> 00:04:01.460


00:04:01.460 --> 00:04:03.330
So now this is an interesting
question.

00:04:03.330 --> 00:04:06.450
So we figured out the molar mass
of our mystery substance

00:04:06.450 --> 00:04:11.680
that took up 98 milliliters and
had a mass of 0.081 grams

00:04:11.680 --> 00:04:14.160
at standard temperature and
pressure, and we figured out

00:04:14.160 --> 00:04:19.170
its molar mass, or its mass per
mole, is 18.5 grams. So

00:04:19.170 --> 00:04:23.700
any guess as to what molecule
we're dealing with?

00:04:23.700 --> 00:04:25.630
18.5 grams.

00:04:25.630 --> 00:04:28.990
And probably it's not going to
be exact, but at least in my

00:04:28.990 --> 00:04:31.680
brain, water seems to
be a good candidate.

00:04:31.680 --> 00:04:33.390
Water is H20.

00:04:33.390 --> 00:04:35.370
Maybe I should do it in blue
because it's water.

00:04:35.370 --> 00:04:38.030


00:04:38.030 --> 00:04:40.260
The mass of each
hydrogen is 1.

00:04:40.260 --> 00:04:43.700
Remember, hydrogen, at least
in its most standard form,

00:04:43.700 --> 00:04:45.610
doesn't have a neutron, so it's
really just a proton and

00:04:45.610 --> 00:04:48.810
and an electron, so it has an
atomic mass of 1 or a molar

00:04:48.810 --> 00:04:50.290
mass of 1 gram.

00:04:50.290 --> 00:04:53.700
And oxygen has a molar mass of
16 grams. So you have two

00:04:53.700 --> 00:04:57.720
hydrogens, so it's 2 plus
16 is equal to 18.

00:04:57.720 --> 00:05:01.970
So it looks like our mystery
substance is water.

00:05:01.970 --> 00:05:02.939