Dr. Joe Burnett
Spring 2010
Chem 177
Exam II
March 3, 2010
This exam consists of
13 questions on 9 pages
Answer Key
Name______________________
Recitation TA ______________
Recitation Section ___________
Grading
Teaching Assistants and Recitation Sections
Page
Points
Score
2
24 pts
________
3
14 pts
________
4
24 pts
________
5
14 pts
________
6
16 pts
________
7
12 pts
TOTAL
104 pts
Name
Stacey Althaus
Sections
5
Time
10:00
Kyle Marchuk
1, 10
8:00, 1:10
Kristopher McKee
11, 12
2:10, 1:10
Justin Smith
8
12:10
Anthony Stender
2, 3
8:00, 9:00
Nishad Thamban Chandrika
4, 6
9:00, 11:00
Songchen Xu
7, 9
11:00, 12:10
________
Questions are written on both sides of each page. There is a periodic table and an equation on the last page.
You may remove this page for scratch paper. Show all work. Answers should include appropriate units and
contain the correct number of significant figures.
The last day to drop Chem 177 and add Chem 50 is Thursday, March 4, at 4:00 PM. See your advisor to obtain
the drop/add form and go to the General Chemistry Office, room 1608 Gilman Hall.
2
1.
(2 pts) Which of the following is a strong electrolyte?
a) HCl(aq)
b) H2O
c) acetic acid
d) ammonia
e) all of the above are strong electrolytes
2.
(6 pts) Consider an aqueous solution of acetic acid, write the formulas of all species expected to be
present (other than H2O). Circle the substance present at the highest concentration in the solution.
C2H3O–(aq)
3.
(4 pts) What is the oxidation state of the sulfur atom in the metabisulfite ion S2O52–?
2x + 5 (–2) = –2
4.
2x – 10 = –2
2x = +8
x = +4
(4 pts) Write the reaction equation that represents what happens when ammonia NH3 is dissolved in
water. [Hint: what type of aqueous solution results from the dissolution of NH3?] For full credit,
indicate the phases of all reactants and products (aq, l, s, g).
NH3(aq) + H2O(l)
NH3(aq) + H2O(l)
5.
H+(aq) or H3O+(aq)
C2H3O2H(aq)
NH4OH(aq) or
NH4+(aq) + OH–(aq)
(8 pts) How many liters of 0.23 molar NaOH is needed to neutralize 2.63 liters of 0.49 molar EDTA.
EDTA is a weak acid that can donate four protons H+ and is symbolized in the following equation as
H4Y. For full credit, show all work, use units, and report your answer to the correct number of
significant figures.
4 NaOH(aq) + H4Y
⎛0.49 mol H4Y⎞
⎟
2.63 L H4Y ⎜⎝
⎠
1L
4 H2O(l) + Na4Y(aq)
⎛4 mol NaOH⎞ ⎛
⎞
1L
⎜
⎟ ⎜
⎟
⎝ 1 mol H4Y ⎠ ⎝0.23 mol NaOH⎠ = 22 L (2 sig figs)
3
6.
(14 pts) In 1847, the Italian scientist Ascanio Sobrero first synthesized nitroglycerin, the explosive
agent in dynamite. Nitroglycerin C3H5(NO3)3 is produced by the reaction of glycerin C3H5(OH)3 and
nitric acid:
C3H5(OH)3 + 3 HNO3
C3H5(NO3)3 + 3 H2O
What is the theoretical yield in grams of nitroglycerin that can be formed
when 251 g of glycerin C3H5(OH)3 is added to 2.00 liters of 6.00 Molar
HNO3(aq)? For full credit, show all work, use units, and report your final
answer to the correct number of significant figures.
Limiting reactant:
theoretical yield of nitroglycerin
glycerin
619 g (3 sig figs)
grams
⎛1 mol glycerin⎞ ⎛1 mol nitroglycerin⎞ ⎛
⎞
227 g
⎟ ⎜
⎟ ⎜
⎟
251 g glycerin ⎜⎝
⎠
⎝
⎠
⎝
92.1 g
1 mol glycerin
1 mol nitroglycerin⎠ =
⎛6.00 mol HNO3⎞ ⎛1 mol nitroglycerin⎞ ⎛
⎞
227 g
⎟ ⎜
⎟ ⎜
⎟
2.0 L ⎜⎝
⎠
⎝
⎠
⎝
1L
3 mol HNO
1 mol nitroglycerin⎠ = 908 g
3
substance
molar mass
g/mol
C3H5(OH)3
glycerin
HNO3
nitric acid
C3H5(NO3)3
nitroglycerin
H2 O
water
619 g (3 sig figs)
92.1
63.0
227
18.0
4
7.
Nitroglycerin is a shock sensitive explosive and decomposes according to the following reaction:
4 C3H5(NO3)3
A.
ΔH = –5811 kJ
(4 pts) This reaction is ___________ .
a) endothermic
B.
12 CO2(g) + 10 H2O(g) + 6 N2(g) + O2(g)
b) exothermic
c) neither exothermic nor endothermic
(2 pts) This reaction is ___________ reaction.
a) a redox or oxidation–reduction
b) metathesis
c) a neutralization
d) a combination
e) none of the above
C.
(4 pts) When nitroglycerin detonates under typical room temperature and pressure conditions,
________________ .
a) work is done by the reaction on the surroundings
b) work is done on the reaction by the surroundings
c) no work occurs
D.
(2 pts) For the decomposition of nitroglycerin, the magnitude of ΔH is ____ than that of ΔE.
a) greater than
E.
b) less than
c) equal to
(4 pts) Briefly but completely explain your answer to the last question.
ΔE = q + w; q is negative (exothermic) and w is negative (work done on the surroundings). The
reaction transfers heat energy to the surroundings and expends energy in doing work on the
surroundings. Therefore the magnitude of ΔE is greater than ΔH. Both heat and work transfer
for the reaction have energy transferred by the reaction on the surroundings
F.
(8 pts) Calculate the heat transferred (q) when 454 g of nitroglycerin decomposes. Report your
answer in kilojoules kJ. For full credit show all work, use units, and report your answer to the
correct number of significant figures. Reporting your answer with the correct sign will be
awarded 1 extra credit point.
⎛1 mol nitroglycerin⎞ ⎛
⎞
–5811 kJ
4
⎟ ⎜
⎟
454 g ⎜⎝
⎠
⎝
227 g
4 mol nitroglycerin⎠ = –2.91 × 10 kJ (3 sig figs)
answer:
–2.91 × 104 kJ (3 sig figs)
kJ
5
8.
(8 pts) Calculate the molar concentration of potassium ions K+ in a solution formed from mixing
10.0 mL of 0.10 Molar K3PO4(aq) with 10.0 mL of 0.10 Molar K2SO4(aq). Assume that the volumes
are additive. For full credit, show all work, use units, and report your answer to the correct number of
significant figures. Please report your final answer in the box.
moles of K+ in 10.0 mL of 0.10 Molar K3PO4:
⎛0.10 mol K3PO4⎞ ⎛ 3 mol K+ ⎞
10.0 mL ⎜⎝ 1000 mL ⎟⎠ ⎜⎝1 mol K PO ⎟⎠ = 0.00300 mol K+
3
4
moles of K+ in 10.0 mL of 0.10 Molar K2SO4:
⎛0.10 mol K2SO4⎞ ⎛ 2 mol K+ ⎞
10.0 mL ⎜⎝ 1000 mL ⎟⎠ ⎜⎝1 mol K SO ⎟⎠ = 0.00200 mol K+
2
4
total volume in liters: 10.0 mL + 10.0 mL = 20.0 mL
molarity of K+:
⎛ 1 L ⎞
20.0 mL ⎜⎝1000 mL⎟⎠ = 0.0200 L
0.00300 mol + 0.00200 mol 0.00500 mol
= 0.0200 L = 0.25 Molar (2 sig figs)
0.0200 L
[K+(aq)] = 0.25 Molar (2 sig figs)
9.
(6 pts) When 1.00 g of H2 reacts with 1.00 g of O2 to form water, O2 is the limiting reactant. What is the
mass amount of excess H2 in grams? For full credit, show all work, use units, and report your answer to
the correct number of significant figures. 2 H2(g) + O2(g)
2 H2O(l)
⎛1 mol O2⎞ ⎛2 mol H2⎞ ⎛ 2.02 g ⎞
Amount of H2 that reacts: 1.0 g O2 ⎜⎝ 32.0 g ⎟⎠ ⎜⎝1 mol O ⎟⎠ ⎜⎝1 mol H ⎟⎠ = .126 g H2
2
2
Excess H2: 1.00 g – 0.126 g = 0.87 g H2 (2 sig figs)
answer:
0.87 g H2 (2 sig figs)
grams
6
10.
11.
(12 pts) Complete and balance the following reaction equations. For full credit, include the phase of all
reacting substances (s, l, g, or aq). If no reaction is expected to occur, write, “no reaction.”
A.
(NH4)2S(aq) + 2 AgNO3(aq)
Ag2S(s) + 2 NH4NO3(aq)
B.
HCl(aq)
no reaction
C.
H2SO4(aq)
+
NaNO3(aq)
+ 2 KOH(aq)
K2SO4(aq)
+ 2 H2O(l)
(4 pts) Write the net ionic equation for the following reaction. For full credit, include the phase of all
substances (s, l, g, or aq).
2 HClO3 (aq) +
Ba(OH)2 (aq)
H+(aq) + OH–(aq)
H2O(l)
Ba(ClO3)2 (aq) + 2 H2O(l)
7
12.
(8 pts) When 50.0 mL of 1.00 Molar HNO3 and 50.0 mL of 1.00 Molar NaOH are combined in a
constant pressure calorimeter, the temperature increases by 6.69oC. The total solution mass is 100.0 g
and you may assume that the solution has the same specific heat as water, 4.18 J/g-oC.
What is the enthalpy change of the reaction: HNO3(aq) + NaOH(aq)
H2O(l) + NaNO3(aq)?
Note that the moles of HNO3 and NaOH that are combined are identical and stoichiometric; exactly
enough HNO3 is added to react with the NaOH.
50.0 mL of a 1.00 Molar solution contains 0.0500 moles of solute.
For full credit, show all work, and report your answer to the correct number of significant figures.
J
qsolution = 100.0 g × 4.18 g oC × 6.69oC = 2.796 × 103 J
qrxn = –2.796 × 103 J
⎛ 1 kJ ⎞
heat in kJ: –2.796 × 103 J ⎜⎝1000 J⎟⎠ = –2.796 kJ
⎛ –2.79 kJ ⎞
ΔHrxn = 1 mol HNO3 or NaOH ⎜⎝0.05006 mol⎟⎠ = –55.9 kJ
(3 sig figs)
answer:
–55.9 kJ
13.
(3 sig figs)
kJ
(4 pts) Using the following information
2 CO(g)
2 C(s) + O2(g)
ΔH = +222 kJ
What is the enthalpy of reaction of the following reaction?
1
C(s) + 2 O2(g)
CO(g)
ΔH = ?
For full credit show all work, use units, and report your answer to the correct number of significant
figures.
2 C(s) + O2(g)
2 CO(g)
1
C(s) + 2 O2(g)
CO(g)
ΔH = –222 kJ
ΔH =
–222 kJ
= –111 kJ
2
answer:
–111 kJ
(3 sig figs)
kJ
8
Chem 177
Periodic Table
Exam II
9
1 cal = 4.184 joules
q
specific heat = sp. ht. = mass in g × ΔT
q = sp.ht. × m × ΔT
= cs × m × ΔT
ΔE = – CvΔT
density of water at 25oC: 1.00 g/mL
J
specific heat of water: 4.18 g oC
Substance Molar Mass
H2
2.02 g/mol
O2
32.0 g/mol
H2 O
18.0 g/mol