1
00:00:00,000 --> 00:00:00,560


2
00:00:00,560 --> 00:00:04,379
I've taken this problem from Chapter 4 of the Chemistry &

3
00:00:04,379 --> 00:00:09,519
Chemical Reactivity book by Kotz, Treichel and Townsend,

4
00:00:09,519 --> 00:00:11,000
and I've done it with their permission.

5
00:00:11,000 --> 00:00:13,070
So let's do this example.

6
00:00:13,070 --> 00:00:20,190
A 1.034 gram sample of impure oxalic acid is dissolved in

7
00:00:20,190 --> 00:00:26,440
water and an acid-base indicator added.

8
00:00:26,440 --> 00:00:33,859
The sample requires 34.47 milliliters of 0.485 molar

9
00:00:33,859 --> 00:00:37,259
sodium hydroxide to reach the equivalence point.

10
00:00:37,259 --> 00:00:41,960
What is the mass of oxalic acid, and what is its mass

11
00:00:41,960 --> 00:00:43,500
percent in the sample?

12
00:00:43,500 --> 00:00:46,520
So before we even break into the math of this, let's just

13
00:00:46,520 --> 00:00:47,520
think about what's happening.

14
00:00:47,520 --> 00:00:51,370
We have some oxalic acid, which looks like this.

15
00:00:51,369 --> 00:00:54,929
It's really two carbolic acid groups joined together, if

16
00:00:54,929 --> 00:00:56,289
that means anything to you.

17
00:00:56,289 --> 00:00:58,509
Watch the organic chemistry play list if you want to learn

18
00:00:58,509 --> 00:00:59,799
more about that.

19
00:00:59,799 --> 00:01:03,119
So we have a double bond to one oxygen, and then another

20
00:01:03,119 --> 00:01:04,259
bond to a hydroxide.

21
00:01:04,260 --> 00:01:06,250
We have that on the other carbon as well.

22
00:01:06,250 --> 00:01:08,840


23
00:01:08,840 --> 00:01:11,240
This right here is what oxalic acid is.

24
00:01:11,239 --> 00:01:14,039
And it's an interesting acid, because it can actually donate

25
00:01:14,040 --> 00:01:14,900
two protons.

26
00:01:14,900 --> 00:01:18,550
This proton can be nabbed off, and this proton can also be

27
00:01:18,549 --> 00:01:19,149
contributed.

28
00:01:19,150 --> 00:01:20,445
And it's actually resident stabalized.

29
00:01:20,444 --> 00:01:22,579
If that doesn't mean anything to you, don't worry.

30
00:01:22,579 --> 00:01:25,500
You'll learn more about that in organic chemistry.

31
00:01:25,500 --> 00:01:27,700
But the important thing to realize here is that there's

32
00:01:27,700 --> 00:01:30,000
two protons to nab off of it.

33
00:01:30,000 --> 00:01:33,409


34
00:01:33,409 --> 00:01:38,409
Now each molecule of sodium hydroxide-- remember when you

35
00:01:38,409 --> 00:01:42,060
put it in the water it really just dissolves, and you can

36
00:01:42,060 --> 00:01:46,140
really just think of it as hydroxide-- each molecule of

37
00:01:46,140 --> 00:01:52,400
hydroxide can nab one of the hydrogen protons.

38
00:01:52,400 --> 00:01:59,490
So for every one molecule of oxalic acid, you're going to

39
00:01:59,489 --> 00:02:03,949
need two hydroxides-- one to nab this hydrogen proton, and

40
00:02:03,950 --> 00:02:06,260
then another one to nab that hydrogen proton.

41
00:02:06,260 --> 00:02:10,000
So let's write down the balanced equation that we're

42
00:02:10,000 --> 00:02:11,120
dealing with here.

43
00:02:11,120 --> 00:02:13,689
So we're going to start off with some oxalic acid.

44
00:02:13,689 --> 00:02:18,270
So that has two hydrogens-- so it's H2-- two carbons, and

45
00:02:18,270 --> 00:02:21,270
then four oxygens-- O4.

46
00:02:21,270 --> 00:02:24,520
It's dissolved in water, so it's an aqueous solution.

47
00:02:24,520 --> 00:02:27,360
And to that, we're going to add sodium hydroxide.

48
00:02:27,360 --> 00:02:31,950


49
00:02:31,949 --> 00:02:34,369
Now I just told you that you're going to need two of

50
00:02:34,370 --> 00:02:37,789
the hydroxides to fully neutralize the oxalic acid.

51
00:02:37,789 --> 00:02:39,199
So you're going to need two of them.

52
00:02:39,199 --> 00:02:42,359
And this is also in our aqueous solution.

53
00:02:42,360 --> 00:02:46,650
And once the reaction happens, this guy will have lost both

54
00:02:46,650 --> 00:02:50,569
of the hydrogen protons, so let me draw that.

55
00:02:50,569 --> 00:02:51,810
So it will look like this.

56
00:02:51,810 --> 00:02:55,219
No more hydrogen, so it's C2O4.

57
00:02:55,219 --> 00:02:57,340
It'll have a negative 2 charge.

58
00:02:57,340 --> 00:03:01,280
And actually, you could imagine that it might be

59
00:03:01,280 --> 00:03:05,099
attracted to these positively charged sodiums. And 2 sodiums

60
00:03:05,099 --> 00:03:05,759
in particular.

61
00:03:05,759 --> 00:03:07,399
So this has a negative 2 charge.

62
00:03:07,400 --> 00:03:10,920
We could even write it there if you want-- 2 minus.

63
00:03:10,919 --> 00:03:14,229
And then you could have the sodiums over here.

64
00:03:14,229 --> 00:03:16,789
You have these two sodiums that have two plus.

65
00:03:16,789 --> 00:03:19,060
And this entire molecules becomes neutral.

66
00:03:19,060 --> 00:03:22,180
They are attracted to each other.

67
00:03:22,180 --> 00:03:25,650
They are still in an aqueous solution.

68
00:03:25,650 --> 00:03:30,129
And then, the hydroxide nabs the protons, and then you are

69
00:03:30,129 --> 00:03:31,799
left with just water.

70
00:03:31,800 --> 00:03:36,130
So plus 2 moles-- or 2 molecules depending on how

71
00:03:36,129 --> 00:03:42,199
we're viewing this-- plus two waters.

72
00:03:42,199 --> 00:03:43,989
I'll just use that same orange color.

73
00:03:43,990 --> 00:03:46,270
Plus two H2Os.

74
00:03:46,270 --> 00:03:52,860
One of the hydrogens in each of the water molecules are

75
00:03:52,860 --> 00:03:57,030
coming from the oxalic acid, and so two of these hydrogens

76
00:03:57,030 --> 00:03:59,909
in these two moles of the water are coming from one

77
00:03:59,909 --> 00:04:02,819
entire molecule of oxalic acid.

78
00:04:02,819 --> 00:04:06,549
Now let's actually do the math.

79
00:04:06,550 --> 00:04:11,610
We have 34.47 milliliters of the solution that has the

80
00:04:11,610 --> 00:04:12,460
sodium hydroxide.

81
00:04:12,460 --> 00:04:15,560
And I'm just going to convert that to liters just so it's

82
00:04:15,560 --> 00:04:18,629
easier to deal with the molarity right over there.

83
00:04:18,629 --> 00:04:26,810
So we have 34.47 milliliters-- we could write it of the

84
00:04:26,810 --> 00:04:28,949
solution, but we understand that, that's the case.

85
00:04:28,949 --> 00:04:37,289
So let's just So this is times, we have one liter for

86
00:04:37,290 --> 00:04:41,569
every 1,000 milliliters.

87
00:04:41,569 --> 00:04:45,300
And then this will give us-- the milliliters cancel out--

88
00:04:45,300 --> 00:04:57,889
34.47 divided by 1,000 is 0.03447 liters of this 0.485

89
00:04:57,889 --> 00:05:00,699
molar sodium hydroxide solution.

90
00:05:00,699 --> 00:05:03,409
So let's figure out how many actual molecules of sodium

91
00:05:03,410 --> 00:05:05,000
hydroxide we have. This is the solution.

92
00:05:05,000 --> 00:05:10,939
And we know its concentration, 0.485 molar-- so let me do

93
00:05:10,939 --> 00:05:14,740
that in a different color-- 0.485 molar, this information

94
00:05:14,740 --> 00:05:16,460
allows us to figure out the actual

95
00:05:16,459 --> 00:05:18,589
molecules of sodium hydroxide.

96
00:05:18,589 --> 00:05:30,829
So we want to multiply this by-- we have 0.485 moles of

97
00:05:30,829 --> 00:05:38,379
sodium hydroxide for every 1 liter of this solution.

98
00:05:38,379 --> 00:05:40,310
That's what the molarity tells us.

99
00:05:40,310 --> 00:05:43,259
We have 0.485 moles per liter.

100
00:05:43,259 --> 00:05:45,740
So the liters cancel out, and then now we're going to

101
00:05:45,740 --> 00:05:47,949
actually have to get a calculator out.

102
00:05:47,949 --> 00:05:50,620
And this'll tell us how many moles of sodium hydroxide we

103
00:05:50,620 --> 00:05:51,689
have in this solution.

104
00:05:51,689 --> 00:05:54,689
So let me get my calculator.

105
00:05:54,689 --> 00:05:55,980
There we go.

106
00:05:55,980 --> 00:05:58,860
All right, let me just multiply these two numbers.

107
00:05:58,860 --> 00:06:13,580
So we have 0.03447 times 0.485 is equal to-- let me put this

108
00:06:13,579 --> 00:06:16,079
down here-- 0.167.

109
00:06:16,079 --> 00:06:19,889
And we only have three significant digits here, so

110
00:06:19,889 --> 00:06:22,189
we're going to round to three significant digits.

111
00:06:22,189 --> 00:06:27,629
So we'll just go with 0.0167.

112
00:06:27,629 --> 00:06:31,250
So let me move that over off the screen.

113
00:06:31,250 --> 00:06:41,569
So this is going to be equal to 0.0167, and all we have

114
00:06:41,569 --> 00:06:44,050
left here are moles of sodium hydroxide.

115
00:06:44,050 --> 00:06:48,069


116
00:06:48,069 --> 00:06:53,620
Now we know that this many moles of sodium hydroxide are

117
00:06:53,620 --> 00:06:58,660
going to completely react with however many moles of oxalic

118
00:06:58,660 --> 00:07:03,300
acid we have. Now we know that we need two moles of this for

119
00:07:03,300 --> 00:07:05,170
every mole of oxalic acid.

120
00:07:05,170 --> 00:07:08,879
Or for every mole of oxalic acid that completely reacts,

121
00:07:08,879 --> 00:07:10,259
we need two moles of this.

122
00:07:10,259 --> 00:07:11,509
So let's write that down.

123
00:07:11,509 --> 00:07:13,870


124
00:07:13,870 --> 00:07:14,680
And then you color.

125
00:07:14,680 --> 00:07:21,730
So we need two moles of sodium hydroxide, we got that from

126
00:07:21,730 --> 00:07:25,509
our balanced equation right there, and it's obvious it

127
00:07:25,509 --> 00:07:28,659
needs one mole, or one molecule will take this

128
00:07:28,660 --> 00:07:30,080
proton, and then you need another molecule

129
00:07:30,079 --> 00:07:31,159
to take that proton.

130
00:07:31,160 --> 00:07:37,180
So we need two moles of sodium hydroxide for every one mole

131
00:07:37,180 --> 00:07:39,030
of oxalic acid.

132
00:07:39,029 --> 00:07:41,039
For every one mole of H2C2O4.

133
00:07:41,040 --> 00:07:43,770


134
00:07:43,769 --> 00:07:45,500
So essentially, we are just going to divide

135
00:07:45,500 --> 00:07:46,810
this number by 2.

136
00:07:46,810 --> 00:07:49,089
Let me get the calculator back.

137
00:07:49,089 --> 00:07:58,339
So we're just going to divide 0.0167 divided by 2.

138
00:07:58,339 --> 00:08:03,154
Once again, three significant digits 0.00835.

139
00:08:03,154 --> 00:08:12,049
So this is going to 0.00835 moles of oxalic acid, H2C2O4.

140
00:08:12,050 --> 00:08:15,410


141
00:08:15,410 --> 00:08:17,010
So we have the number of moles, but we to

142
00:08:17,009 --> 00:08:18,789
figure out the mass.

143
00:08:18,790 --> 00:08:23,590
And we know the molar mass of oxalic acid.

144
00:08:23,589 --> 00:08:24,579
Let me write these down.

145
00:08:24,579 --> 00:08:29,194
We know that hydrogen has a molar mass-- let me write it

146
00:08:29,194 --> 00:08:34,490
this way-- molar mass if you have a mole of hydrogen, it

147
00:08:34,490 --> 00:08:37,460
has a molar mass of one gram.

148
00:08:37,460 --> 00:08:41,370
If you have-- and this comes from its atomic weight-- if

149
00:08:41,370 --> 00:08:49,950
you have carbon its molar mass is 12 grams. And if you have

150
00:08:49,950 --> 00:08:56,509
oxygen, its molar mass is 16 grams.

151
00:08:56,509 --> 00:08:59,710
So what's the molar mass of oxalic acid?

152
00:08:59,710 --> 00:09:02,410
Well we have two hydrogens, so that's going to

153
00:09:02,409 --> 00:09:04,639
be two grams, right?

154
00:09:04,639 --> 00:09:05,830
2 times 1 gram.

155
00:09:05,830 --> 00:09:07,340
That's the hydrogens there.

156
00:09:07,340 --> 00:09:12,629
We have two carbons, so it's going to be plus 24 grams. 12

157
00:09:12,629 --> 00:09:14,679
grams for each of these carbons.

158
00:09:14,679 --> 00:09:17,799
And then we're going to have four oxygens that weigh in, if

159
00:09:17,799 --> 00:09:19,969
we have a mole of them, at 16 grams. So that's

160
00:09:19,970 --> 00:09:22,850
going to be plus 64.

161
00:09:22,850 --> 00:09:23,960
So what does this come out to?

162
00:09:23,960 --> 00:09:30,870
24 plus 64 is 88, right?

163
00:09:30,870 --> 00:09:32,200
2 plus 6 is 88, right.

164
00:09:32,200 --> 00:09:40,420
So it's 88 plus 2 more is 90 grams. So if you had a mole of

165
00:09:40,419 --> 00:09:43,969
oxalic acid, it would be 90 grams. So we could say 90

166
00:09:43,970 --> 00:09:52,180
grams per mole of H2C2O4.

167
00:09:52,179 --> 00:09:55,429


168
00:09:55,429 --> 00:09:59,879
So let's go back to the math here.

169
00:09:59,879 --> 00:10:01,789
I'll rewrite it over here.

170
00:10:01,789 --> 00:10:08,699
We know we're dealing with 0.00835 moles of

171
00:10:08,700 --> 00:10:10,129
oxalic acid, H2C2O4.

172
00:10:10,129 --> 00:10:12,700


173
00:10:12,700 --> 00:10:16,009
And now we know its molar mass.

174
00:10:16,009 --> 00:10:20,159
We know that there are 90 grams-- let me do this in a

175
00:10:20,159 --> 00:10:23,429
different color, this color's getting motonous-- we know

176
00:10:23,429 --> 00:10:30,120
that there are 90 grams of H2C2O4 for

177
00:10:30,120 --> 00:10:36,679
every one mole of H2C2O4.

178
00:10:36,679 --> 00:10:37,870
This is its molar mass.

179
00:10:37,870 --> 00:10:40,529
And now we just multiply this number, and we'll figure out

180
00:10:40,529 --> 00:10:42,789
the grams of oxalic acid.

181
00:10:42,789 --> 00:10:45,199
That and that cancels out.

182
00:10:45,200 --> 00:10:45,379
and.

183
00:10:45,379 --> 00:10:48,490
Then we just take the number that we had and multiply it by

184
00:10:48,490 --> 00:10:50,860
90, so times 90.

185
00:10:50,860 --> 00:10:53,879
This just says the previous answer, which is the number of

186
00:10:53,879 --> 00:10:57,509
moles of oxalic acid times its molar mass will tell us the

187
00:10:57,509 --> 00:10:59,860
grams of oxalic acid.

188
00:10:59,860 --> 00:11:01,909
So we get 0.75.

189
00:11:01,909 --> 00:11:03,750
I'll just round it to 2 since we only have

190
00:11:03,750 --> 00:11:05,000
three significant digits.

191
00:11:05,000 --> 00:11:07,659


192
00:11:07,659 --> 00:11:11,860
The 90 isn't exact, so it's a little bit-- but I'll just

193
00:11:11,860 --> 00:11:13,570
round it to three significant digits.

194
00:11:13,570 --> 00:11:17,940
So 7--.752.

195
00:11:17,940 --> 00:11:27,770
This is equal to 0.752 grams of oxalic acid: H2C2O4.

196
00:11:27,769 --> 00:11:30,519
And I think we've answered part of the question.

197
00:11:30,519 --> 00:11:33,389
So the first question is, what is the mass of oxalic acid?

198
00:11:33,389 --> 00:11:34,799
We've just answered it.

199
00:11:34,799 --> 00:11:39,639
That answer right there is 0.752 grams.

200
00:11:39,639 --> 00:11:41,980
Now the next part is, what is its mass

201
00:11:41,980 --> 00:11:43,600
percent in the sample?

202
00:11:43,600 --> 00:11:47,600
Well the sample of impure oxalic acid right over here,

203
00:11:47,600 --> 00:11:52,950
was 1.034 grams. So we just have to say, what percentage

204
00:11:52,950 --> 00:11:57,060
is 0.752 of 1.034?

205
00:11:57,059 --> 00:11:59,250
So let's get the calculator back.

206
00:11:59,250 --> 00:12:14,580
So we have the 0.752 divided by 1.034 and we get 72.7%.

207
00:12:14,580 --> 00:12:18,150
So the answer to the second part right

208
00:12:18,149 --> 00:12:21,220
over there is, 72.7%.

209
00:12:21,220 --> 00:12:24,170
We were able to figure out that this impure oxalic acid

210
00:12:24,169 --> 00:12:29,990
sample is 72.7% actual oxalic acid.

211
00:12:29,990 --> 00:12:30,198