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ANSWER KEY
The Hong Kong University of Science and Technology
DEPARTMENT OF CHEMISTRY
Last Name:
_________________________________
First Name: __________________________________
Student ID: __________________________________
CHEM 1030 General Chemistry II
Midterm Examination
Date: Wednesday, April 17th, 2013
Time Allowed: 120 minutes
Total Points:
100 points
v.1
1
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Part I. Multiple Choices [2 points each, 50 points total].
For each question, select the most appropriate answer by writing the letter out in the empty box
provided.
1.1
1.2.
Which of the following is/are oxidation-reduction reactions?
I.
PCl3 + Cl2 → PCl5
II.
Cu + 2AgNO3 → Cu(NO3)2 + 2Ag
III.
CO2 + 2 LiOH → Li2CO3 + H2O
IV.
FeCl2 + 2NaOH → Fe(OH)2 + 2NaCl
A)
B)
C)
D)
E)
III
IV
I and II
I, II, and III
I, II, III, and IV
A correct statement of Henry's law is:
A)
B)
C)
D)
E)
1.3.
Which of the following solutions has the lowest boiling point?
A)
B)
C)
D)
E)
1.4
The concentration of a gas in solution is inversely proportional to temperature.
The concentration of a gas in solution is directly proportional to the mole fraction
of solvent.
The concentration of a gas in solution is independent of pressure.
The concentration of a gas in a solution is inversely proportional to pressure.
None of these.
0.15 M Na2S
0.10 M CaBr2
0.15 M Ba(NO3)2
0.20 M C6H12O6 (glucose)
Both A and C
When rubidium metal is exposed to air, two atoms of rubidium (Rb) combine with one
atom of oxygen. If 1.46 grams of rubidium is exposed to air and reacted to completion,
what will be the mass of the product in grams?
A)
B)
C)
D)
E)
17.46 g
1.60 g
9.46 g
6.39 g
3.19 g
2
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1.5
The dihydrogenphosphate ion, H2PO4–, has both a conjugate acid and a conjugate base.
These are, respectively:
A)
B)
C)
D)
E)
1.6
The limiting reactant in a reaction
A)
B)
C)
D)
E)
1.7
has the lowest coefficient in a balanced equation
is the reactant for which you have the fewest number of moles
has the largest coefficient in a balanced equation
is the reactant that produces the largest amount of products when reacted
completely
none of these
Which of the following do you need to know to be able to calculate the molarity of a salt
solution?
I.
II.
III .
IV.
A)
B)
C)
D)
E)
1.8
H3PO4, PO43–
HPO42–, H3PO4
H2PO4–, HPO42–
HPO42–, PO43–
H3PO4, HPO42–
the mass of salt added
the molar mass of the salt
the volume of water added
the total volume of the solution
I, III
I, II, III
II, III
I, II, IV
You need all of the above information.
The correct mathematical expression for finding the molar solubility (x) of Sn(OH)2 is:
A)
B)
C)
D)
E)
2x2 = Ksp
2x3 = Ksp
108x5 = Ksp
4x3 = Ksp
8x3 = Ksp
3
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1.9
The empirical formula of a group of compounds is CHCl. Lindane, a powerful insecticide,
is a member of this group. The molar mass of lindane is 290.8 g/mol. How many atoms
of carbon does a molecule of lindane contain?
A)
B)
C)
D)
E)
1.10
You have immersed the electrodes of a light bulb into a solution of H2SO4 such that the
light bulb is on (see figure below). You add a solid substance into the solution and the
bulb grows dim. Which of the following solid substance could have been added?
A)
B)
C)
D)
E)
F)
1.11
Ba(OH)2
NaNO3
K2SO4
Cu(NO3)2
Both B and C
Both A and D
Which of the following mixtures would result in a buffered solution?
A)
B)
C)
D)
E)
1.12
2
3
4
6
8
Mixing 100.0 mL of 0.100 M HCl with 100.0 mL of 0.100 M NaOH.
Mixing 100.0 mL of 0.100 M NH3 with 100.0 mL of 0.100 M NaOH.
Mixing 100.0 mL of 0.100 M NH3 with 100.0 mL of 0.100 M HCl.
Mixing 100.0 mL of 0.100 M NH3 with 50.0 mL of 0.100 M HCl.
None of the above mixtures would result in a buffered solution.
A salt solution sits in an open beaker. Assuming constant temperature and pressure, the
vapor pressure of the solution
A)
B)
C)
D)
E)
increases over time
decreases over time
stays the same over time
need to know which salt is in the solution to answer this
need to know the temperature and pressure to answer this
4
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1.13
Sodium chloride is added slowly to a solution that is 0.010 M in Cu+, Ag+, and Au+. The
Ksp values for the chloride salts are 1.9 × 10–7, 1.6 × 10–10, and 2.0 × 10–13, respectively.
Which compound will precipitate first?
A)
B)
C)
D)
E)
1.14
Which of the following species cannot act as a Lewis base?
A)
B)
C)
D)
E)
F)
1.15
S2–
NH2–
BH3
H2O
PH3
Two of the above cannot act as Lewis base
Two liquids form a solution and release a quantity of heat. How does the pressure above
the solution compare to that predicted by Raoult's law?
A)
B)
C)
D)
E)
1.16
CuCl
AgCl
AuCl
All will precipitate at the same time.
Cannot be determined.
It will be greater.
It will be less.
It will be the same.
It will show positive deviation.
None of these.
Consider a solution consisting of the following two buffer systems:
H2CO3
H2PO4–
HCO3– + H+ pKa = 6.4
HPO42– + H+ pKa = 7.2
At pH 6.4, which one of the following is true of the relative amounts of acid and
conjugate base present?
A)
B)
C)
D)
E)
[H2CO3] > [HCO3–] and [H2PO4–] > [HPO42–]
[H2CO3] = [HCO3–] and [H2PO4–] > [HPO42–]
[H2CO3] = [HCO3–] and [HPO42–] > [H2PO4–]
[HCO3–] > [H2CO3] and [HPO42–] > [H2PO4–]
[H2CO3] > [HCO3–] and [HPO42–] > [H2PO4–]
5
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1.17
A non-volatile solute added to a solvent raises the boiling point of the solution because:
A)
B)
C)
D)
E)
1.18
Which of the following statements is/are true?
A)
B)
C)
D)
E)
1.19
The pH of a strong acid solution is always lower than the pH of a weak acid
solution.
The pH of a solution can never be negative.
For a conjugate acid-base pair, Ka = 1/Kb.
At least two of the statements A-C are true.
All of the statements A-C are false.
Solid calcium hydroxide is dissolved in 2.00 L of water until the pH of the solution is
10.00. Assuming no volume change, how many moles of calcium hydroxide is added to
the solution?
A)
B)
C)
D)
E)
1.20
The temperature to cause boiling must be great enough to boil not only the
solvent but also the solute.
The solute particles lower the solvent's vapor pressure, thus requiring a higher
temperature to cause boiling.
The solute particles raise the solvent's vapor pressure, thus requiring a higher
temperature to cause boiling.
The solute increases the volume of the solution, and an increase in volume
requires an increase in the temperature to reach the boiling point (derived from
PV = nRT).
Two of the above are correct.
2.0 x 10-4 mol
5.0 x 10-5 mol
1.0 x 10-10 mol
1.0 x 10-4 mol
2.0 x 10-10 mol
A 7.0 × 10–3 M solution of a weak acid is 5.1% dissociated at 25 oC. In a 7.0 × 10–4 M
solution, the percentage of dissociation would be
A)
B)
C)
D)
E)
the same
> 5.1%
< 5.1%
zero
need to know the Ka of the acid to answer this
6
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1.21
Given the following values of equilibrium constants:
AgC2H3O2(s)
Ag+(aq) + C2H3O2–(aq)
Ag+(aq) + 2NH3(aq)
Ag(NH3)2+(aq)
Ksp = 1.9 × 10–3
K = 1.7 × 107
What are the effects on the solubility of AgC2H3O2 by adding either the acid HNO3 or the
base NH3?
A)
B)
C)
D)
E)
1.22
When a non-volatile solute is added to a volatile solvent, the solution vapor pressure
__________, the boiling point __________, the freezing point __________, and the
osmotic pressure across a semipermeable membrane __________.
A)
B)
C)
D)
E)
1.23
decreases, increases, decreases, decreases
increases, increases, decreases, increases
increases, increases, increases, decreases
decreases, increases, increases, decreases
decreases, increases, decreases, increases
For a solution equimolar in HCN and NaCN, which statement is false?
A)
B)
C)
D)
E)
F)
1.24
Either substance would decrease the solubility.
NH3 would increase the solubility, but HNO3 would decrease it.
NH3 would increase the solubility, but HNO3 would have virtually no effect.
Either substance would increase the solubility.
NH3 would decrease the solubility, but HNO3 would increase it.
This is an example of the common ion effect.
The [H+] is larger than it would be if only the HCN was in solution.
The [H+] is almost equal to the Ka.
Addition of more NaCN will shift the acid dissociation equilibrium of HCN to the
left.
Addition of NaOH will increase [CN–] and decrease [HCN].
Two of these are false
In the titration of a weak acid, HA, with a sodium hydroxide solution of approximately the
same concentration, the equivalence point occurs at pH = 10. Which of the following
weak acid indicators would be best suited to mark the endpoint of this titration?
A)
B)
C)
indicator A, Ka = 10–11
indicator B, Ka = 10–13
indicator C, Ka = 10–9
D)
E)
F)
indicator D, Ka = 10–7
indicator E, Ka = 10–5
Both A and C
7
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1.25
What is the best way to ensure complete precipitation of SnS from a saturated H2S
solution?
A)
B)
C)
D)
E)
Add more H2S.
Add a strong acid.
Add a weak acid.
Add a strong base.
Add a weak base.
Part II. Short Questions: [50 points]
Clearly write your answers in the space provided.
2.1
2.2
Balance the following chemical equations using smallest whole numbers: [3 points]
a)
___ NH3 + ___ O2 → ___ NO + ___ H2O
(4, 5, 4, 6)
b)
___ I2O5 + ___ H2S → ___ I2 + ___ SO2 + ___ H2O
(3, 5, 3, 5, 5, Ch 10. Redox balancing)
A 1.90 g sample of an oxide of bromine is converted to 3.188 g of AgBr (molar mass of
AgBr = 187.78 g/mol).
Determine the empirical formula of the oxide. [3 points]
Ans: BrO2
Empirical Formula:
______________
8
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2.3
Consider the following reaction: [4 points]
2A + B → 3C + D
where the molar mass of A is 15.0 g/mol;
the molar mass of B is 25.0 g/mol;
the molar mass of C is 10.0 g/mol.
a)
3.0 mol A and 2.0 mol B react to form 4.0 mol C. What is the percent yield of this
reaction?
Ans: 89%
Percent Yield:
___________
b)
What is the molar mass of D?
Ans: 25.0 g/mol
Molar Mass of D:
_____________
_
2.4
2.5
Consider the reaction between saturated aqueous solutions of ZnSO4(aq) and CaBr2(aq):
[2 points]
a)
Determine the net ionic equation, if any occurs.
Ca2+ (aq) + SO42- (aq) → CaSO4 (s)
b)
List any spectator ions.
Zn2+, Br-
A 9.61 M solution of H2SO4 in water has a density of 1.520 g/mL.
Calculate the molality of H2SO4. [3 points]
Ans: 16.6 m
9
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Molality of H2SO4:
______________
2.6
Given the following volatile liquids: [2 points]
Methanol (CH3OH)
Benzene (C6H6)
Chloroform (CH3Cl)
Water (H2O)
a)
Which two liquids, when mixed, would show the most positive deviation to the
Raoult’s Law?
Ans: Water and Benzene
b)
Which two liquids, when mixed, would show the most negative deviation to the
Raoult’s Law?
Ans: Water and methanol
2.7
A 10.40 g solid mixture of table sugar (C12H22O11) and table salt (NaCl) is dissolved in
150. g of water, the freezing point of the solution is found to be –2.24 oC. (For water: Tf =
0 oC, Kf = 1.86 oC kg/mol).
Assuming ideal behavior and that NaCl is completely dissociated, calculate the mass
percent composition of table sugar in the original solid mixture. [5 points]
Ans: 53.9% wt
10
Mass % of table sugar:
_______________
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2.8
Given the following data: [2 points]
HC2H3O2
HF
HCN
a)
Ka = 1.8 x 10-5
Ka = 7.2 x 10-4
Ka = 6.2 x 10-10
Which of the following salt is most basic?
Circle your choice.
KBr
NaF
NaCN
KC2H3O2
Ans: NaCN
2.9
Consider a 4.00 M solution of a triprotic acid, H3A (Ka1 = 5.0 x 10-3; Ka2 = 8.0 x 10-8; Ka3 =
6.0 x 10-10). [3 points]
a)
Determine the pH of this solution at equilibrium.
Ans: 0.85
pH: _______
b)
Determine the equilibrium concentration of [H3A]eq.
Ans: [H3A] = 3.86 M
[H3A]eq: _______
11
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2.10
Given their pKa values, predict and sketch the chemical structures of the following acids:
[4 points]
H3AsO3
(pKa1 = 9.2)
Ans:
H3PO4 (pKa1 = 2.1)
Ans:
2.11
Consider a 1.0-liter buffered solution made from 0.47 M NH3 and 0.26 M NH4F (Kb = 1.8
x 10-5). [4 points]
a)
What is the pH of this solution?
[OH-] = 3.25 x 10-5 M pOH = 4.49
pH = 9.51
pH: _______
b)
What is the pH when 0.10 mole of NaOH(s) is added to this solution?
NH3 → NH4+ + OH0.57 0.16M
[OH- ] = 6.4 x 10-5 M pOH = 4.19
12
pH = 9.81
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pH: _______
2.12
Consider the following HOAc/NaOAc buffer solutions (Ka of HOAc = 1.8 x 10-5):
[2 points]
Buffer Solution A:
Buffer Solution B:
2.13
5.0 M HOAc and 5.0 M NaOAc
5.0 x 10-7 M HOAc and 5.0 x 10-7 M NaOAc
a)
Which buffer solution would have the lower pH? Ans: Solution A
b)
At equal volume, which buffer solution would have the greater buffering capacity?
Ans: Solution A
A 100.0-mL sample of 0.200 M (CH3)3N (Kb = 5.28 × 10–5) is titrated with 0.200 M HCl.
[5 points]
a)
What is the volume of HCl required to reach the equivalence point?
Ans: 100. mL
Volume of HCl:
____________
b)
What is the pH at the equivalence point?
Ans: 5.36
pH: _______
c)
Which of the following indicators would be best for this titration? (the pH range of
the indicator is shown in brackets)
Circle your choice.
i.
ii.
Crystal violet (0.2 – 1.8)
Alizarin yellow (10.1 – 12.0)
13
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iii.
iv.
v.
2.14
Phenolphthalein (8.2 – 10.0)
Methyl red (4.8 – 6.0)
Methyl orange (3.2 – 4.4)
Given that all of the following ionic salts have a Ksp value of 1.0 x 10-10 in water, at 25 oC.
[2 points]
AB2
AB
A2B
AB3
A3B2
A3B
a) Which ionic salt would have the greatest water solubility?
Ans: A3B2
b) Which ionic salt would have the lowest water water solubility? Ans: AB
2.15
Consider an aqueous solution mixture consisting of: [6 points]
350. mL of 3.2 M Pb(NO3)2
200. mL of 0.020 M NaCl
Given that the Ksp of lead chloride is 1.6 × 10–5.
a)
Will precipitation occur? Briefly explain.
Ans: Yes, since Q (1.1 x 10-4) > Ksp (1.6 x 10-5), precipitation will occur until Q =
Ksp
b)
Determine the equilibrium concentration of the chloride ion.
Ans: 1.4 × 10–3 M
14
- THE END OF MIDTERM -
[Cl- ]eq: __________