Chemistry 3202
Test Unit 2 Section 3
Page 1 of 7
CDLI - Chemistry 3202 Unit 2 Section 3
Acids and Bases
2006
Part I - Multiple Choice: Using the form on page 4, circle the letter of the best response.
1.
Which statement is true regarding titrations?
A. The indicator changes color at the equivalence point
B. The pH at the equivalence is always 7
*C. The equivalence point occurs when the moles of acid and base are equal
D. The burette must always be filled to read an initial reading of 0.00mL
2.
What is the correct burette reading?
A. 17.20 mL
*B. 17.25 mL
C. 18.75 mL
D. 18.80 mL
3.
The graph shows how the pH of a HCl solution varied as it was
titrated with a KOH solution. What volume of KOH(aq) was
required to reach the endpoint of the titration?
pH
A. 26 mL
*B. 27 mL
C. 28 mL
D. 30 mL
4.
26 27 28
30
volume of KOH
What is the best way to minimize random error in a titration experiment?
*A. perform the experiment several times and average the trials
B. ensure all equipment is clean and dry
C. have different people perform the experiment
D. ensure all chemicals are of the highest purity
Chemistry 3202
Page 2 of 7
Which best represents the titration of 50.00 mL HCl(aq) with 0.100 mol/L NaOH(aq)?
14
12
10
8
6
4
2
0
pH
pH
5.
Test Unit 2 Section 3
10
20
30
14
12
10
8
6
4
2
0
40
10
Volume titrant added
40
14
12
10
8
6
4
2
0
pH
pH
B.
10
20
30
*C.
14
12
10
8
6
4
2
0
10
40
20
30
40
Volume titrant added
Volume titrant added
D.
A titration is performed using a pH meter with the reactants below. What will be the pH at
the equivalence point?
HF(aq) + KOH(aq) →
A. acidic
*B. basic
C. neutral
D. cannot tell from information given
7.
30
Volume titrant added
A.
6.
20
Which indicator is yellow in an aqueous solution that has a pH of 10.0?
*A. methyl red
B. phenol red
C methyl violet
D. thymol blue
Chemistry 3202
8.
Test Unit 2 Section 3
Page 3 of 7
What would be the color of a 1.0 mol/L solution of HCl (aq) in an indicator mixture consisting
of phenol red and thymolpthalein?
A. blue
B. colorless
C. red
*D. yellow
9.
A solution is tested using the indicators listed. What is the approximate pH of the solution?
A. 2.5
B. 5.5
*C. 6.5
D. 8.5
Indicator
Methyl orange
phenolpthalein
Phenol red
litmus
Color
Yellow
Colorless
Yellow
Blue
10. Which indicator should be used to detect the first equivalence point?
A. indigo carmine
B. methyl orange
C. methyl violet
*D. phenolphthalein
11.
Which is dibasic?
A. CN*B. CO32C. HSO4D. PO43-
12. What is the approximate pH of a solution resulting from mixing 40.0 mL of 0.0010 M HCl (aq)
and 60.0 mL pf 0.0050 M NaOH (aq) ?
A. 3
B. 5
C 7
*D. 13
Chemistry 3202
Test Unit 2 Section 3
Page 4 of 7
13. Aqueous sulfuric acid is neutralized by the addition of an excess of sodium hydroxide. What
sulfur containing molecule or ion is present in largest concentration when the reaction is
complete?
A.
B.
C.
*D.
HSO4H2SO4
Na2S
SO42-
14. Solid NaOH is a deliquescent solid which means it becomes hydrated and we have no
accurate way of knowing the degree of hydration. What process must be carried out before
we can use a NaOH solution in a titration experiment?
A. neutralization with a solution of HCl
B. neutralization with a solution of KHSO3
*C. standardization with a primary standard such as potassium hydrogen phthalate, KHP
D. standardization with a primary standard solution of Na2CO3
15. Which part of this curve is [HPO42- ] most abundant when phosphoric acid is titrated with
sodium hydroxide?
A.
B.
*C.
D.
I
II
III
IV
16. Which titration pair would have the lowest pH at the equivalence point?
*A. HBr with NH3
B. HCl with KOH
C. HNO2 with LiOH
D. HNO3 with NaOH
Chemistry 3202
Test Unit 2 Section 3
Page 5 of 7
Name:________________________________________School:________________________________
Chemistry 3202 Unit 2 Section 3 -- Answer Sheet
Part 1: For each item, circle the letter corresponding to your choice.
1.
A
B
C
D
7.
A
B
C
D
13.
A
B
C
D
2.
A
B
C
D
8.
A
B
C
D
14.
A
B
C
D
3.
A
B
C
D
9.
A
B
C
D
15.
A
B
C
D
4.
A
B
C
D
10.
A
B
C
D
16.
A
B
C
D
5.
A
B
C
D
11.
A
B
C
D
6.
A
B
C
D
12.
A
B
C
D
Part II: Constructed Responses: Complete each item in the space provided. State final
answers in sentence form and follow the rules for rounding/significant digits.
17. A student pipets 10.0 mL of 0.150 mol/L oxalic acid, HOOCCOOH(aq) (a diprotic acid), into
a 250 mL erlenmeyer flask and titrates it with a sodium hydroxide solution. On average,
22.14 mL of sodium hydroxide is used to completely neutralize the oxalic acid to its second
equivalence point.
{2}
a) Sketch a pH curve for the titration.
{2}
b) Write a net ionic equation for each step in the curve and write the overall equation.
1) HOOCCOOH + OH − → HOOCCOO − + HOH
2) HOOCCOO − + OH − → OOCCOO2− + HOH
overall:
HOOCCOOH + 2OH − → OOCCOO 2− + 2HOH
Chemistry 3202
Test Unit 2 Section 3
Page 6 of 7
Name:________________________________________School:________________________________
{2}
17
c) Calculate the concentration of the sodium hydroxide solution.
n HOOCCOOH = Cv = 0.150 mol L x 0.01000L = 0.00150 mol HOOCCOOH
mole ratio 2:1
n NaOH = 2 ( n HOOCCOOH ) = 2 ( 0.00150 mol ) = 0.00300 mol NaOH
C NaOH =
{3}
n 0.00300 mol
=
= 0.136 mol L NaOH
v
0.02214L
18. A student carefully mixes 25.0 mL of 0.0520 M nitric acid (HNO3 (aq) ) with 15.0 mL of
0.0830 M potassium hydroxide (KOH(aq)). Calculate the final pH of the mixture.
HNO3 (aq ) + KOH → KNO3 + HOH
Find moles of H3O + and OH - :
n H O+ = Cv = 0.0520 M x 0.0250L = 0.00130 mol HNO3 = H3O +
3
n OH− = Cv = 0.0830 M x 0.0150 L = 0.00125 mol KOH = OH H 3O + + OH − → HOH + HOH
thus the H3O + is in excess!
n H O+
3
excess
= 0.00130 − 0.00125 = 0.0000500 mol = 5.00 x 10-5 mol H 3O+
pH = − log(H3O + ) = − log(5.00 x 10-5 ) = 4.30
{1}
19. a) Why does HCl have to be standardized when used in titration experiments?
Hydrochloric acid is produced by dissolving HCl(g) in water. Over time the dissolved gas
evapourates from the water. As a result the HCl(aq) becomes weaker over time! (ie: just like a
can of pop going flat after it has been opened)
Since it becomes weaker over time we will be unsure of its exact concentration at the time we
are ready to use it. Therefore we must standardize the HCl(aq) solution, that is, we must perform
a titration experiment to determine its exact concentration.
Chemistry 3202
Test Unit 2 Section 3
Page 7 of 7
Name:________________________________________School:________________________________
{1}
b) What substance can be used to standardize hydrochloric acid?
To standardize a solition of HCl we must titrated it against a primary standard. A primary standard is a
substance that does not hydrate. When a mass of primary standard is measured out we can be
absolutely sure that it is 100% pure (no water), thus we can accurately prepare the standard solution and
be certain of its concentration. We will use the primary standard as the titrant and the HCl as the sample.
Stoichiometry on the result will then yield the certain concentration of our HCL. A primary standard that
could be used to standardize HCL is Na2CO3(s).
20. Bromphenol blue, HBp is an acid-base indicator with an approximate pH range of 3.0 - 4.6.
Bromphenol blue indicator is yellow below pH 3.0 and pink above pH 4.6
{1}
a) Write an ionization equation for bromophenol blue.
HBp(aq)
yellow
+
H2O(l)
⇌
+
H3O(aq)
+
−
Bp(aq)
pink
{1}
b) If a drop of this indicator is added to a 1.0 M HCl solution. What color should be
observed?
A 1.0 mol/l solution of Hydrochloric acid will have a pH of 0.0 thus is below 3.0 so the
indicator will be in the HBp form and turn the solution yellow.
{1}
c) Describe how to shift the position of the indicator equilibrium to favour the conjugate
base form of the indicator. Explain in terms of Le chateliers principle.
In order to shift this to the conjugate base form Bp- (the pink form) sufficient amounts of
base must be added to neutralize the hydrochloric acid. The OH- will react with and
remove the H3O+. This will cause the equilibrium system to respond by shifting forward to
replace the lost H3O+