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Hi I'm Jocelyn and today we're
going to go over fall 2009

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exam 1, problem 1.

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00:00:29,450 --> 00:00:30,920
So to begin with, we
always want to

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00:00:30,920 --> 00:00:33,110
read the whole problem.

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00:00:33,110 --> 00:00:37,020
So in this one it says uranium
metal can be produced by the

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reaction of uranium
tetrafluoride with magnesium

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in a sealed reactor heated
to 700 degrees C.

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00:00:44,526 --> 00:00:47,790
So we know that we're dealing
with a chemical reaction here.

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00:00:47,790 --> 00:00:50,350
The byproduct is magnesium
fluoride.

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00:00:50,350 --> 00:00:53,170
To ensure that all the magnesium
is consumed in the

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00:00:53,170 --> 00:00:56,320
reaction, the reactor is charged
with excess uranium

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00:00:56,320 --> 00:01:00,070
fluoride, specifically 10% more
than the stoichiometric

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requirement of the reactor.

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To produce 222 kilograms of
uranium, how much uranium

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tetrafluoride must be introduced
into the reactor?

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00:01:09,490 --> 00:01:12,020
So that's what the question
is asking us.

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00:01:12,020 --> 00:01:15,320
Then it says we assume complete
conversion of

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magnesium, express your
answer in kilograms.

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00:01:20,150 --> 00:01:27,470
So we know that the question is
asking us for the uranium

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00:01:27,470 --> 00:01:36,065
tetrafluoride needed to make
222 kilograms of uranium.

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Right?

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00:01:37,570 --> 00:01:40,240
That's the last sentence.

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00:01:40,240 --> 00:01:42,630
The sentence that's before that,
however, also gives us

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very important information.

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00:01:44,410 --> 00:01:50,400
It tells us that we need
the stoichiometric

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00:01:50,400 --> 00:01:54,130
equivalent plus 10%.

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00:01:54,130 --> 00:01:57,390
And that's very important to
keep in mind because, as

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00:01:57,390 --> 00:02:00,260
you'll see later, that's where a
lot of people got tripped up

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00:02:00,260 --> 00:02:02,850
on this problem.

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00:02:02,850 --> 00:02:06,930
So then let's go to the first
two sentences in this problem

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00:02:06,930 --> 00:02:09,890
and extract the information
given there.

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00:02:09,890 --> 00:02:12,070
We know that we're dealing
with a chemical reaction.

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00:02:12,070 --> 00:02:14,870
So when we are dealing with one
of those we always want to

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00:02:14,870 --> 00:02:18,090
put down the chemical
equation.

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00:02:18,090 --> 00:02:22,530
So we have uranium tetrafluoride
reacting with

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00:02:22,530 --> 00:02:28,330
magnesium to give uranium
with a byproduct

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00:02:28,330 --> 00:02:33,150
of magnesium fluoride.

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00:02:33,150 --> 00:02:36,220
And it tells us that it that
it's at 700 degrees C.

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00:02:36,220 --> 00:02:40,610
That's superfluous information
but you can always put

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00:02:40,610 --> 00:02:44,600
reaction conditions above the
arrow just to keep track.

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00:02:47,610 --> 00:02:49,940
So now that we have this
equation are we

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00:02:49,940 --> 00:02:52,420
ready to move on?

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00:02:52,420 --> 00:02:52,585
No.

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00:02:52,585 --> 00:02:53,410
Right?

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00:02:53,410 --> 00:02:55,970
We need to have a mass
conservation.

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We need to balance
this equation.

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00:02:58,110 --> 00:03:00,990
So to determine if this is
balanced or not, we want to

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00:03:00,990 --> 00:03:05,130
keep track of the amount of
uranium, fluoride and

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00:03:05,130 --> 00:03:08,420
magnesium on each side
of the equation.

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00:03:08,420 --> 00:03:14,840
So we have uranium, fluoride
and magnesium.

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00:03:14,840 --> 00:03:17,350
And then we have on
the product side--

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00:03:20,890 --> 00:03:26,620
and that's not right.

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00:03:26,620 --> 00:03:28,451
This is the reactant
side, right?

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00:03:31,280 --> 00:03:33,494
And over here is the
product side.

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00:03:37,440 --> 00:03:44,270
So on the reactant side we have
1 uranium, 4 fluoride and

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1 magnesium.

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On the product side we
have 1 uranium, 2

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00:03:51,100 --> 00:03:53,750
fluoride and 1 magnesium.

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00:04:01,120 --> 00:04:05,030
So we see that we have an
imbalance in fluoride.

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00:04:05,030 --> 00:04:09,420
To fix that we can put a
stoichiometric coefficient in

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00:04:09,420 --> 00:04:12,370
front of the magnesium fluoride
on the product side

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00:04:12,370 --> 00:04:14,910
in order to have 4 fluorides.

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00:04:14,910 --> 00:04:16,800
We put a 2 there.

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00:04:16,800 --> 00:04:18,260
And we change that to 2.

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00:04:18,260 --> 00:04:19,990
But we also have to change
the magnesium.

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00:04:19,990 --> 00:04:21,240
Right?

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00:04:23,960 --> 00:04:26,870
Now, although we fixed the
problem in the fluoride, we

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00:04:26,870 --> 00:04:29,400
have an imbalance in
the magnesium.

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00:04:29,400 --> 00:04:33,310
So to fix that, we put a
stoichiometric coefficient in

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00:04:33,310 --> 00:04:39,210
front of the magnesium, switch
our accounting down here, and

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00:04:39,210 --> 00:04:41,810
we see that we now have
conservation of mass.

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The two sides are balanced.

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00:04:45,990 --> 00:04:51,460
So with our balanced equation we
can now move on to what the

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00:04:51,460 --> 00:04:54,450
question is actually
asking us.

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00:04:54,450 --> 00:05:02,180
And we know that we have,
we want to make 222

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00:05:02,180 --> 00:05:03,850
kilograms of uranium.

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00:05:08,360 --> 00:05:10,380
We then need to figure
out how much uranium

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00:05:10,380 --> 00:05:12,470
tetrafluoride is needed.

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00:05:12,470 --> 00:05:16,990
This chemical reaction
gives us mole ratios.

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00:05:16,990 --> 00:05:20,310
But we are, right now, have
kilograms. So the first thing

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00:05:20,310 --> 00:05:25,180
we need to do is convert
kilograms to moles.

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00:05:25,180 --> 00:05:27,490
And we do that using
stoichiometry or unit

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00:05:27,490 --> 00:05:29,390
conversion.

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00:05:29,390 --> 00:05:32,900
So the other thing we
need to know is the

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00:05:32,900 --> 00:05:34,325
molar mas of uranium.

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00:05:34,325 --> 00:05:36,480
So this is something you can
look up on your periodic

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00:05:36,480 --> 00:05:41,410
table, online, whatever
resources you have available.

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00:05:41,410 --> 00:05:57,160
So the molar mass of uranium
is 238 grams per mole.

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00:05:57,160 --> 00:06:00,220
Now we just do a simple unit
conversion to get the moles

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00:06:00,220 --> 00:06:01,490
uranium that we want.

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00:06:05,290 --> 00:06:14,330
So first we need to convert to
grams. That's 1,000 grams. And

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00:06:14,330 --> 00:06:19,560
then using the molar mass,
we convert to moles.

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00:06:24,710 --> 00:06:29,820
And because I have grams up
top here I need to put the

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00:06:29,820 --> 00:06:34,444
molar grams down here in order
to cancel everything out.

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00:06:38,160 --> 00:06:50,200
And that gives us 933
moles uranium.

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00:06:50,200 --> 00:06:54,050
OK so we have the number of
moles uranium we want to make.

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00:06:54,050 --> 00:06:57,260
Now we can use the chemical
equation over here to

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00:06:57,260 --> 00:07:01,690
determine how much uranium
tetrafluoride we need.

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00:07:01,690 --> 00:07:05,610
So the chemical equation tells
us that we need a 1:1 ratio of

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00:07:05,610 --> 00:07:08,210
uranium tetrafluoride.

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00:07:08,210 --> 00:07:11,580
However this is where we need to
go back to what the problem

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00:07:11,580 --> 00:07:13,510
was actually asking us.

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00:07:13,510 --> 00:07:16,470
And remember that we need the
stoichiometric equivalent,

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00:07:16,470 --> 00:07:21,760
which is given by this
mole ratio, plus 10%.

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00:07:21,760 --> 00:07:29,850
So the amount of uranium
tetrafluoride we need is the

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00:07:29,850 --> 00:07:38,560
stoichiometric equivalent plus
10% times that stoichiometric

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00:07:38,560 --> 00:07:39,810
equivalent.

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00:07:44,440 --> 00:07:53,830
And that equals 1,026 moles
of uranium tetrafluoride.

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00:07:56,350 --> 00:07:59,590
So we found the amount of
uranium tetrafluoride we need.

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00:07:59,590 --> 00:08:01,680
Are we done?

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00:08:01,680 --> 00:08:01,845
No.

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00:08:01,845 --> 00:08:02,690
Right?

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00:08:02,690 --> 00:08:05,000
If we go back to the question
we see that it asks for our

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00:08:05,000 --> 00:08:08,360
answer expressed in kilograms.
So that's why it's always

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00:08:08,360 --> 00:08:11,370
important to keep track of what
the question is actually

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00:08:11,370 --> 00:08:14,440
asking you and to make sure
you've answered that.

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00:08:14,440 --> 00:08:17,510
So we have uranium tetrafluoride
in moles.

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00:08:17,510 --> 00:08:21,450
Now we need to convert it to
kilograms. Again we need the

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00:08:21,450 --> 00:08:23,520
molar mass.

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00:08:23,520 --> 00:08:26,970
This time of uranium
tetrafluoride.

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00:08:26,970 --> 00:08:28,700
And it equals--

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00:08:28,700 --> 00:08:36,200
so we have 1 uranium per
molecule and 4 fluoride.

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00:08:36,200 --> 00:08:39,590
And again we just look up that
molar mass of fluoride on the

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00:08:39,590 --> 00:08:40,840
periodic table.

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00:08:43,170 --> 00:08:51,126
And that equals 314
grams per mole.

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00:08:55,020 --> 00:08:57,930
So we do our unit conversion
again.

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00:08:57,930 --> 00:09:03,310
And this is my favorite way of
doing stoichiometry unit

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00:09:03,310 --> 00:09:04,380
conversions.

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00:09:04,380 --> 00:09:09,470
You may have another way that
you learned in high

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school or later on.

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00:09:12,170 --> 00:09:16,570
But this way I know that I'm
keeping track of my units and

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00:09:16,570 --> 00:09:19,160
everything's canceling
out correctly.

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00:09:19,160 --> 00:09:25,420
So we have 1,026 moles uranium
tetrafluoride.

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00:09:25,420 --> 00:09:28,403
We use the molar mass
we just determined.

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00:09:32,100 --> 00:09:34,960
And remember that this is an
industrial scale process, so

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00:09:34,960 --> 00:09:45,460
we actually care about the
amounts in kilograms. And we

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00:09:45,460 --> 00:09:56,420
get 322 kilograms uranium
tetrafluoride.

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00:09:56,420 --> 00:09:59,350
Going back to what the question
was asking us, we

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00:09:59,350 --> 00:10:02,310
know that we now have
the correct answer.

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00:10:02,310 --> 00:10:04,300
And we can put a
box around it.

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00:10:04,300 --> 00:10:08,860
So that on the exam whoever's
grading you knows that that is

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00:10:08,860 --> 00:10:10,220
your answer.