#8 Homework holcombe (51160)
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1
10.0 points
A 1 M solution of NaOH is used to titrate a
1 M solution of C6H5COOH (benzoic acid). If
5
the KA of C6H5COOH is 5 10 , what is the
pH of the solution at the equivalence point?
1
2. 0.006 M
3. 0.012 M
4. 0.050 M
Explanation:
VHCL = 25 mL
VNAOH = 40 mL = 0.04 L
MHCL = 0.012 M
The base is NaOH. To neutralize, mol
+
H = mol OH .
1. 7
0.012 mol
2. 5
NH+ =
(25 mL HCl)
L
3. 13
1L
1 mol H
+
1000 mL 1 mol HCl
+
= 0.0003 mol H = NOH = NNAOH
4. not enough information
5. 9 correct
Explanation:
Equal volumes of the titrant and analyte
will be used to reach the equivalance point.
Regardless of the starting volume, at the
equivalance point the volume will be double
the starting value and all of the benzoic acid
will have been converted to benzoate. The
solution will be 0.5 M C6H 5COO .
KB = KW/KA
14
= 10
/(5
5
10
10 ) = 2 10
1/2
[OH ] = (KBCB)
10
= (2 10
1/2
0.5)
10 1/2
5 = (10 )
= 10
pOH = 5
pH = 9
002
10.0 points
It was found that 25 mL of 0.012 M HCl
neutralized 40 mL of NaOH solution. What
was the molarity of the base solution?
1. 0.0075 M correct
M
=
NAOH
mol
0.0003 mol NaOH
0.04 L
L=
= 0.0075 M NaOH
3
10.0 points
What is the pH when 100 mL of 0.1 M HCl
is titrated with 50 mL of 0.2 M NaOH?
1. The pKA of NAOH needs to be
provided to answer this question.
2. The pKA of HCl needs to be provided
to answer this question.
3. pH < 7
4. pH > 7
5. pH = 7 correct
Explanation:
004 (part 1 of 5) 10.0 points
Consider the following titration curve.
Homework #8 holcombe (51160)
2
by inspection of the buer region, or by the
pH at half the equivalence point volume. The
equivalence point is at 70 mL added base,
so the pKa is at 35 mL, about pH=7.5.
14
12
10
006 (part 3 of 5) 10.0 points
What is the pH at the equivalence
(stoichio-metric) point?
8
pH
6
1. about 5
4
2. about 7.5
2
3. about 10 correct
0
0
20 40 60 80 100 120 140
Volume of base (mL)
This is a plot of a (weak, strong) acid
and a (weak, strong) base titration.
1. weak; strong correct
2. strong; weak
3. strong; strong
Explanation:
The curve shape follows that for a weak
acid titrated with a strong base, identified by
the concave down-to-concave up shape of
the curve before the equivalence point.
005 (part 2 of 5) 10.0 points
What is the approximate pKA of the acid?
1. 13
2. 12.5
3. 7.5 correct
4. There is no pKA because it is a
strong acid.
5. 10
6. 5
Explanation:
The pKA of the strong acid can be estimated
4. exactly 7
Explanation:
The equivalence point is the vertical
part of the curve, which is for this curve
the second inflection point (changes from
concave up to concave down) at pH = 10.
007 (part 4 of 5) 10.0 points
What type of salt is produced as a result
of this acid/base reaction?
1. basic correct
2. acidic
3. neutral
Explanation:
The salt is formed from the
neutralization of a weak acid and a
strong base, so it is a basic salt.
008 (part 5 of 5) 10.0 points
Of those listed, which indicator would be
best for this titration?
1. methyl orange, (pKIN=3.4)
2. phenol red, (pKIN=7.9)
3. phenolphthalein, (pKIN=9.4) correct
4. methyl red, (pKIN=5.0)
Explanation:
Homework #8 holcombe (51160)
Phenolphthalein has a pKIN range that
overlaps the pH at the equivalence point.
Alizarin yellow R may be a better choice
based on pKIN, but is not listed.
009
10.0 points
For the titration of 50.0 mL of 0.020 M aqueous salicylic acid with 0.020 M KOH(aq), calculate the pH after the addition of 55.0 mL of
KOH(aq). For salycylic acid, pKA = 2.97.
1. 11.26
3
2. Methyl red, color change
red/yellow at 4.4 < pH < 6.2
3. Any of these is suitable.
4. Phenolphthalein, color change
colorless/red-violet 8.0 < pH < 10.0
correct
5. Neutral red, color change
red/yellow 6.8 < pH < 8.0
Explanation:
The endpoint of the weak acid
(CH3COOH) - strong base (KOH) titration
is above 7. An appropriate indicator has
color change range pH = pKA 1.
2. 10.98 correct
3. 12.30
4. 12.02
5. 7.00
Explanation:
010
10.0 points
Which of the following indicators w ould be
most appropriate to use for the detection
of the equivalence point of the titration of
a weak base such as ammonia with a
strong acid such as HCl?
012
10.0 points
The volume of 0.100 M NaOH required for
the titration of 100 mL of M
CH3COOH 0.100 to the equivalence point would
be (larger than, the same as, smaller than)
if the acid were 100 mL of 0.100 M HCl.
The equivalence point for the HCl titration
would come at (a higher, a lower, the
same) pH than for the CH3COOH titration.
1. smaller than; the same
1. Any of these would be equally useful.
2. larger than; a lower
2. methyl red correct
3. the same; a lower correct
3. phenolphthalein
4. the same as; a higher
4. neutral blue
5. larger than; a higher
Explanation:
The endpoint of a weak base - strong acid
titration will be below pH 7, so methyl red,
pKA 5.5, would be a suitable indicator.
011
10.0 points
Which of the following acid-base indicators
should be used for the titration of CH3COOH
5
with KOH? For acetic acid, KA = 1.8 10 .
1. None of these is suitable.
Explanation:
[NaOH] = 0.1 M
[HCl] = 0.1 M
V
= 100 mL
[CH3COOH] = 0.1 M
VHCL = 100 mL
CH3COOH
The amount of acid required to react a
given number of moles of base depends on
+
the total number of moles of H3O the acid
can provide. 100 mL of 0.1 M CH3COOH and
100 mL of 0.1 M HCl both provide 10.0 mmol
+
H3O . However, because CH3COOH is a
weak acid and HCl is a strong acid the pH at
the equivalence points will dier. In
Homework #8 holcombe (51160)
the case of HCl the spectator ions present
do not hydrolyze and the endpoint pH is 7.
In the case of CH3COOH, the CH3COO
tritra-tion product hydrolyzes, producing
OH ions which raise the pH above 7.
4
1. 10.1 to 12.1
2. 5.1 to 7.1
3. 1.1 to 3.1
013
10.0 points
The acid form of an indicator is yellow and its
anion is blue. The KA of this indicator is 1
5
10 . What will be the approximate pH range
over which this indicator changes color?
4. 8.4 to 10.4
5. 3.6 to 5.6 correct
Explanation:
1. 3 < pH < 5
016
10.0 points
If 100 mL of each of the following solutions
is mixed, which one produces a buer?
2. 8 < pH < 10
3. 9 < pH < 11
1. 1.0 M NH3(aq) + 1.0 M HCl(aq)
4. 4 < pH < 6 correct
2. 1.0 M NH4Cl(aq) + 1.0 M KOH(aq)
5. 5 < pH < 7
3. 1.0 M NH3(aq) + 0.6 M KOH(aq)
Explanation:
The pKA of this indicator is 5, so the
indi-cator will change colors around pH
5. Thus you would expect a color
change between pH 4 and pH 6.
014
10.0 points
2
Calculate the ratio of the molarities of HPO 4
and H2PO 4 ions required to achieve buering
at pH = 7.00. For H3PO4, pKA1 = 2.12, pKA2
= 7.21, and pKA3 = 12.68.
1. 0.21
2. 0.81
3. 1.23
4. 1.62
5. 0.62 correct
Explanation:
015
10.0 points
Choose the e ective pH range of an
aniline-anilinium chloride bu er. The
10
value of the KB for aniline is 4.3 10
.
4. 1.0 M NH4Cl(aq) + 0.4 M HCl(aq)
5. 1.0 M NH3(aq) + 0.4 M HCl(aq)
correct Explanation:
017 (part 1 of 2) 10.0 points
Citric acid is a triprotic acid (pKA1 = 3.13,
pKA2 = 4.76, pKA3 = 6.40). 50 mL of 0.020 M
citric acid is titrated with 0.050 M NaOH.
How many mL of base solution are needed
to reach the first equivalence point?
1. 50.0 mL
2. 2.0 mL
3. 5.0 mL
4. 20.0 mL correct
5. 60.0 mL
Explanation:
50 mL (0.02 M) = 1.0 mmol of acid
This means you need 1 mmol of base
to reach the first equivalence point.
1 mmol of NaOH / 0.05 M = 20.0 mL of
Homework #8 holcombe (51160)
base solution.
5
monoprotic weak acid with a strong base.
018 (part 2 of 2) 10.0 points
Which answer best approximates the pH
of the solution half way to the first
equivalence point of this titration?
pH
1. 8.6
2. 4.0
3. 3.1 correct
Volume of base added
4. 7.0
The pH at point I is equal to the
the pH at point II is
7.
5. 6.4
6. 4.8
1. pKA of the acid; equal to
7. 2.2
and
2. pH of the acid; greater than
Explanation:
At the half-way point to the first equivalence point, the solution pH should
equal pKA1 which is 3.13. The choice of
3.1 is a good approximation.
019
10.0 points
You have a solution that is buered at pH =
2.0 using H3PO4 and H2PO 4 (pKA1 = 2.12;
pKA2 = 7.21; pKA3 = 12.68). You decide to
titrate this buer with a strong base. 15.0 mL
are needed to reach the first equivalence
point. What is the total volume of base that
will have been added when the second
equivalence point is reached?
1. > 30 mL correct
2. < 30 mL
3. A second equivalence point in the
titra-tion will never be observed.
4. 30 mL
Explanation:
020
10.0 points
The figure below shows the titration of a
3. pKA of the acid; less than
4. pH of the acid; less than
5. pKA of the acid; greater than
correct Explanation: