1
00:00:00,000 --> 00:00:00,610


2
00:00:00,610 --> 00:00:03,719
I said I would get you a more interesting mass composition

3
00:00:03,720 --> 00:00:07,099
to empirical formula problem, one that doesn't just have a

4
00:00:07,099 --> 00:00:08,589
straight-up 2:1 ratio.

5
00:00:08,589 --> 00:00:09,879
And so here it is.

6
00:00:09,880 --> 00:00:11,350
I have a bag of stuff.

7
00:00:11,349 --> 00:00:13,160
Or let's call this a bottle of stuff.

8
00:00:13,160 --> 00:00:14,929
Maybe it's in its liquid form.

9
00:00:14,929 --> 00:00:21,929
And it happens to be 2.04% hydrogen, 65.3% oxygen, and

10
00:00:21,929 --> 00:00:25,070
32.65% sulfur.

11
00:00:25,070 --> 00:00:27,949
What is the empirical formula, what's our best stab at the

12
00:00:27,949 --> 00:00:30,239
empirical formula, of this substance?

13
00:00:30,239 --> 00:00:32,478
So what we would do, like we do in all these problems,

14
00:00:32,478 --> 00:00:35,689
let's just assume we've got 100 grams of the stuff.

15
00:00:35,689 --> 00:00:37,839
So we have 100 grams of the stuff.

16
00:00:37,840 --> 00:00:45,400
So we assume 100 grams. Let me do that in a good yellow.

17
00:00:45,399 --> 00:00:52,109
So let's say, assume I have 100 grams. How many grams of

18
00:00:52,109 --> 00:00:53,179
hydrogen do I have?

19
00:00:53,179 --> 00:00:56,079
If I have 100 grams total, 2.04% of that is hydrogen, so

20
00:00:56,079 --> 00:01:00,359
I have 2.04 grams of hydrogen.

21
00:01:00,359 --> 00:01:05,349
I have 65.3 grams of oxygen.

22
00:01:05,349 --> 00:01:13,030
And I have 32.65 grams of sulfur.

23
00:01:13,030 --> 00:01:15,280
Now, what we need to do now is figure out how many moles of

24
00:01:15,280 --> 00:01:16,099
hydrogen is this.

25
00:01:16,099 --> 00:01:17,579
How many moles of oxygen.

26
00:01:17,579 --> 00:01:18,899
And how many moles of sulfur.

27
00:01:18,900 --> 00:01:21,570
Then we can compare the ratios and we should be able to know

28
00:01:21,569 --> 00:01:22,819
the empirical formula.

29
00:01:22,819 --> 00:01:26,059


30
00:01:26,060 --> 00:01:28,079
What is the mass of 1 mole of hydrogen?

31
00:01:28,079 --> 00:01:29,370
Let me write that.

32
00:01:29,370 --> 00:01:36,130
So 1 mole of hydrogen.

33
00:01:36,129 --> 00:01:39,159
Well we know what the mass number for hydrogen is.

34
00:01:39,159 --> 00:01:41,099
It's 1.

35
00:01:41,099 --> 00:01:44,399
And especially, the atomic weight, also for hydrogen, if

36
00:01:44,400 --> 00:01:45,800
we were to take it on Earth.

37
00:01:45,799 --> 00:01:47,119
The composition, you pretty much just

38
00:01:47,120 --> 00:01:49,340
find hydrogen nucleuses.

39
00:01:49,340 --> 00:01:50,520
If it's neutral, it has an electron,

40
00:01:50,519 --> 00:01:52,250
but it has no neutrons.

41
00:01:52,250 --> 00:01:56,219
So it has an atomic mass of one atomic mass unit.

42
00:01:56,219 --> 00:01:58,519
So one mole of hydrogen.

43
00:01:58,519 --> 00:02:01,469
If you have a ton of hydrogens together, or a mole of them,

44
00:02:01,469 --> 00:02:07,929
not a ton, I shouldn't say, you have 6.02 times 10 to the

45
00:02:07,930 --> 00:02:09,360
23 hydrogens.

46
00:02:09,360 --> 00:02:12,590
Then you take hydrogen's atomic mass number in atomic

47
00:02:12,590 --> 00:02:13,050
mass units.

48
00:02:13,050 --> 00:02:15,250
And you say, well, it'll be that many grams

49
00:02:15,250 --> 00:02:17,360
of hydrogen, right?

50
00:02:17,360 --> 00:02:21,230
So if you immediately look up here, if we have 2.04 grams of

51
00:02:21,229 --> 00:02:24,679
hydrogen, how many moles of hydrogen do we have?

52
00:02:24,680 --> 00:02:28,110
Well, one mole is one gram, so we have

53
00:02:28,110 --> 00:02:32,720
2.04 moles of hydrogen.

54
00:02:32,719 --> 00:02:35,460
Notice, this said what the mass of the hydrogen is.

55
00:02:35,460 --> 00:02:39,099
This tells us how many hydrogen molecules we have.

56
00:02:39,099 --> 00:02:44,000
Remember, this is 2.04 times 6.02 times 10 to the 23

57
00:02:44,000 --> 00:02:48,050
hydrogen atoms. Moles of hydrogen.

58
00:02:48,050 --> 00:02:49,410
Maybe I should write that down.

59
00:02:49,409 --> 00:02:50,449
So one mole of hydrogen.

60
00:02:50,449 --> 00:02:51,889
There you go.

61
00:02:51,889 --> 00:02:53,209
And then oxygen.

62
00:02:53,210 --> 00:02:57,200
One mole of oxygen.

63
00:02:57,199 --> 00:03:01,560
Oxygen's mass number, in case you forgot, is 16.

64
00:03:01,560 --> 00:03:03,110
Right there.

65
00:03:03,110 --> 00:03:06,270
Oxygen's mass number is 16.

66
00:03:06,270 --> 00:03:11,740
So one mole of oxygen has a mass of 16 grams. 6.02 times

67
00:03:11,740 --> 00:03:15,650
10 to the 23 oxygen atoms has a mass of 16 grams.

68
00:03:15,650 --> 00:03:18,490
So how many moles do we have here?

69
00:03:18,490 --> 00:03:20,850
Let's see.

70
00:03:20,849 --> 00:03:30,849
So if we take 65.3 grams of oxygen.

71
00:03:30,849 --> 00:03:35,219
And we have 16 grams per mole, so you divide by 16.

72
00:03:35,219 --> 00:03:38,180
It equals 4.08125.

73
00:03:38,180 --> 00:03:39,760
I don't want to get too precise here.

74
00:03:39,759 --> 00:03:40,539
But let me just write that.

75
00:03:40,539 --> 00:03:44,810
4.08 moles of oxygen.

76
00:03:44,810 --> 00:03:46,920
So I'm going to write that in oxygen color.

77
00:03:46,919 --> 00:03:53,869
So I have 4.08 moles of oxygen.

78
00:03:53,870 --> 00:03:55,409
And then finally, sulfur.

79
00:03:55,409 --> 00:03:58,754
What is sulfur's atomic mass?

80
00:03:58,754 --> 00:04:00,629
The one thing I'll never forget about

81
00:04:00,629 --> 00:04:02,460
sulfur is its smell.

82
00:04:02,460 --> 00:04:05,540
There's a city in Louisiana which we used to drive to all

83
00:04:05,539 --> 00:04:07,169
the time -- I think we had some family friends there--

84
00:04:07,169 --> 00:04:08,789
called Port Sulfur.

85
00:04:08,789 --> 00:04:11,639
And they did a lot of sulfur processing there.

86
00:04:11,639 --> 00:04:15,129
And lucky for the residents, at least at the time-- I

87
00:04:15,129 --> 00:04:18,159
apologize if they fixed the issue-- it smelled like

88
00:04:18,160 --> 00:04:20,939
sulfur, which smells like rotten eggs.

89
00:04:20,939 --> 00:04:22,389
But anyway, one mole of sulfur.

90
00:04:22,389 --> 00:04:28,479


91
00:04:28,480 --> 00:04:33,680
So sulfur's atomic mass is 32 atomic mass

92
00:04:33,680 --> 00:04:36,310
units per sulfur atom.

93
00:04:36,310 --> 00:04:41,629
So a whole mole of it is going to have a mass of 32 grams. So

94
00:04:41,629 --> 00:04:44,649
a mole of sulfur-- not a mule.

95
00:04:44,649 --> 00:04:48,409
Maybe I should invent a new unit called the mule.

96
00:04:48,410 --> 00:04:52,230
So a mole of sulfur is 32 grams. So how

97
00:04:52,230 --> 00:04:53,379
many moles do we have?

98
00:04:53,379 --> 00:04:55,730
We have a little bit more than one, but let's be precise

99
00:04:55,730 --> 00:04:58,600
here, because everything else is a little bit of a decimal.

100
00:04:58,600 --> 00:05:09,050
So if we have 32.65 grams of sulfur and we divide by the

101
00:05:09,050 --> 00:05:13,170
number of grams per mole-- divided by 32 grams per mole--

102
00:05:13,170 --> 00:05:25,280
we have 1.02 moles of sulfur.

103
00:05:25,279 --> 00:05:27,589
This was hydrogen up here.

104
00:05:27,589 --> 00:05:30,319
So here, you should hopefully see a pretty good ratio, here.

105
00:05:30,319 --> 00:05:33,689
For every one sulfur atom-- I mean, the ratio worked exactly

106
00:05:33,689 --> 00:05:35,639
out and that's because I did this problem before.

107
00:05:35,639 --> 00:05:38,250
I actually made up this problem before we worked, so I

108
00:05:38,250 --> 00:05:39,860
made it so the numbers worked out.

109
00:05:39,860 --> 00:05:44,210
But one mole of sulfur, for every mole of sulfur, so for

110
00:05:44,209 --> 00:05:47,379
every 6.02 times 10 to the 23 sulphur atoms, you have two

111
00:05:47,379 --> 00:05:48,290
moles of hydrogen, right?

112
00:05:48,290 --> 00:05:49,840
This ratio is 1:2.

113
00:05:49,839 --> 00:05:52,239
Two times 1.02 is 2.04.

114
00:05:52,240 --> 00:05:54,430
And then, for every one mole of sulfur, you have

115
00:05:54,430 --> 00:05:55,930
four moles of oxygen.

116
00:05:55,930 --> 00:05:56,379
Right?

117
00:05:56,379 --> 00:05:59,379
Literally, if you multiply this times four you get 4.08.

118
00:05:59,379 --> 00:06:09,959
So the ratio of hydrogen to sulfur to oxygen is for every

119
00:06:09,959 --> 00:06:13,620
one sulfur, we have two hydrogens and

120
00:06:13,620 --> 00:06:15,930
we have four oxygens.

121
00:06:15,930 --> 00:06:25,110
So the empirical formula of this is H2.

122
00:06:25,110 --> 00:06:28,199
And then we have one sulfur.

123
00:06:28,199 --> 00:06:30,289
And then we have four oxygens.

124
00:06:30,290 --> 00:06:33,770
And this is sulfuric acid, one of the things you would least

125
00:06:33,769 --> 00:06:37,500
like poured on you most of the time.

126
00:06:37,500 --> 00:06:40,230
Anyway, hope you found that useful.

127
00:06:40,230 --> 00:06:40,742