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Chem. 1B Final Practice Test 2 Solutions
Name__________________________________________ Print Neatly. You will lose 1 point if I cannot read your name or perm number.
Student Number _________________________________
If you are sitting next to someone with the same version of the test, you will be moved and lose 5
points.
All work must be shown on the exam for partial credit. Points will be taken off for incorrect or
no units. Calculators are allowed. Cell phones may not be used for calculators. On fundamental
and challenge problems you must show your work in order to receive credit for the problem. If
your cell phone goes off during the exam you will have your exam removed from you.
Fundamentals (of 72 possible)
Problem 1 (of 13 possible)
Problem 2 (of 17 possible)
Problem 3 (of 18 possible)
Problem 4 (of 20 possible)
Multiple Choice (of 60 possible)
Extra Credit (of 5 possible)
Final Total (of 200 possible)
First letter
of last name
2
Fundamental Questions Each of these fundamental chemistry questions is worth 6 points. You must show work to get
credit. Little to no partial credit will be rewarded. Make sure your answer includes the proper
units.
1) 6 pts The equilibrium constant K for the following reaction at 900Β°C is 0.0028.
What is KP at this temperature?
CS2(g) + 4H2(g) β CH4(g) + 2H2S(g)
πΎ =[πΆπ»4][π»2π]2
[πΆπ2][π»2]4 πΎπ =
ππΆπ»4ππ»2π
2
ππΆπ2ππ»2
4
πΆ =π
π=
π
π π
πΎ =
ππΆπ»4
π πππ»2π
2
π 2π2
ππΆπ2
π π
ππ»2
4
π 4π4
= π 2π2ππΆπ»4
ππ»2π2
ππΆπ2ππ»2
4 = π 2π2πΎπ
πΎπ =1
π 2π2πΎ
πΎπ =1
(0.08206 πΏβππ‘ππππβπΎ
)2
(1173πΎ)2(0.0028)
πΎπ = 3.02 Γ 10β7
2) 6 pts When 1 mol of a fuel burns at constant pressure it produces 3452 kJ of heat
and does 11 kJ of work. What are the values of ΞE and ΞH for the
combustion of the fuel? Report your answer in ππ½
πππ
π = β3452 ππ½ π€ = β11 ππ½
βπΈ = π + π€ = β3452 ππ½ + β11 ππ½ = β3463 ππ½ βπ» = π at constant temperature βπ» = β3452 ππ½
3)
6 pts
If ΞH is -12.4 ππ½
πππ and ΞS is 0.91 π½
πππβπΎ what is ΞG at 75Β°C and is the reaction
spontaneous.
βπΊ = βπ» β πβπ = β12.4 ππ½
πππβ (348πΎ) (0.00091
ππ½
πππβπΎ) = β12.7
ππ½
πππ
Since ΞG is negative the reaction is spontaneous.
3
4) 6 pts What is the molar solubility of CaF2 in 0.010 M LiF?
CaF2(s) β Ca2+(aq) + 2F-(aq)
πΎπ π = [πΆπ2+] [πΉβ]2 = π(0.010 + 2π )2 = 4.0 Γ 10β11
Since Ksp is small assume 0.010+2S = 0.010
π(0.010)2 = 4.0 Γ 10β11 π = 4.0 Γ 10β7
Check Assumption 2(4.0Γ10β7)
0.010100% = 0.0080% Good
Molar Solubility S=4.0Γ10-7
5) 6 pts
ΞGΒ°rxn for 2SO2(g) + O2(g) 2SO3(g) is -141.74 ππ½
πππ at 25.00Β°C. What is
ΞGrxn when the partial pressure of each gas is 100. atm? What is the
spontaneous direction of the reaction under these conditions?
βπΊ = βπΊΒ° + π πππ(π)
βπΊ = βπΊΒ° + π πππ (πππ3
2
πππ2
2 ππ2
)
βπΊ = β141.74 ππ½
πππ+ (0.0083145
ππ½
πππβπΎ) (298.15πΎ)ππ (
100.2
100.2 100.)
βπΊ = β153.16 ππ½
πππ
Since ΞG is negative the reaction is spontaneous in the forward direction.
6) 6 pts What is K for the following reaction at 25Β°C?
Sn4+(aq) + 2Ag(s) Sn2+(aq) + 2Ag+(aq) EΒ°=0.65 V
Are there more reactants or products at equilibrium?
Sn4+ + 2e- Sn2+
2(AgAg+ + e-)
πΈΒ° =π π
ππΉππ(πΎ)
0.65 π =(8.3145 π½
πππβπΎ)(298πΎ)
(2)(96,485 πΆπππ
)ππ(πΎ)
ππ(πΎ) = 51
πΎ = 9.7 Γ 1021 More products
Ca2+ F-
Initial 0 0.010 M
Change +S +2S
Equilibrium S 0.010+2S
4
7) 6 pts The preparations of two aqueous solutions are described in the table below.
For each solution, write the chemical formulas of the major species present
at equilibrium. You can leave out water itself.
Write the chemical formulas of the species that will act as acids in the 'acids'
row, the formulas of the species that will act as bases in the 'bases' row, and
the formulas of the species that will act as neither acids nor bases in the
'other' row.
You will find it useful to keep in mind that NH3 is a weak base.
0.5 mol of HNO3 is added to 1.0 L of a 0.5
NH3 solution.
acids: NH4+
bases: none
other: NO3-
0.1 mol of NaOH is added to 1.0 L of a
solution that is 0.3 M in both NH3 and
NH4Br.
acids: NH4+
bases: NH3
other: Na+, Br-
8) 6 pts A solution has a pH of 4.90, what is the pOH, [H+], and [OH-]?
πππ» = 14.00 β ππ» = 14.00 β 4.90 = 9.10
[π»+] = 10βππ» = 10β4.90 = 1.3 Γ 10β5 π
[ππ»β] =1.0 Γ 10β14
1.3 Γ 10β5= 7.7 Γ 10β10π
9) 6 pts What is Ecell for the following reaction at 25Β°C?
I2(s)+ Fe(s)Fe2+(aq, 0.20 M) + 2I-(aq, 0.30 M)
πΈ = πΈΒ° βπ π
ππΉππ(π)
Fe Fe2+ + 2e- EΒ° =0.44 V
I2 + 2e- 2I- EΒ° = 0.54 V
Fe + I2 Fe2+ + 2I- EΒ°=0.98 V
π = [πΉπ2+][πΌβ]2
πΈ = 0.98π β(8.3145 π½
πππβπΎ)(298πΎ)
(2)(96,485 πΆ
πππ)
ππ((0.20)(0.30)2) = 1.03 π
5
10) 6 pts A piece of iron of mass 20.0 g at 100.Β°C is placed in a vessel of negligible
heat capacity but containing 50.7 g of water at 22.0Β°C. Calculate the final
temperature of the water. Assume that there is no energy lost to the
surroundings.
ππππ‘ππ = βππ€ππ‘ππ
π = ππΆβπ
(20.0 π πΉπ) (0.45 π½
ββπ) (ππ β 100. β)
= β(50.7 π π»2π) (4.18 π½
ββπ) (ππ β 22.0β)
ππ = 25β
11)
6 pts
Consider the following exothermic reaction at 25Β°C and 1 atm.
2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)
Circle the correct answer for each of the following quantities: w, ΞH, ΞE, ΞS, and ΞG.
w<0 w>0 w=0
ΞH<0 ΞH>0 ΞH=0
ΞE<0 ΞE>0 ΞE=0
ΞS<0 ΞS>0 ΞS=0
ΞG<0 ΞG>0 ΞG=0
12) 6 pts Identify if the following reactions are redox reactions or not. If they are
redox reactions, specify what is oxidized and what is reduced.
Cu(OH)2(s) + 2HNO3(aq) Cu(NO3)2(aq) + 2H2O(l)
Not a redox reaction
Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)
Redox Reaction
Fe +30 gained electrons
Fe is reduced
C +2 +4 lost electrons
C is oxidized
6
Challenge Questions
Each of the following short answer questions are worth the noted points. Partial credit will be
given. Make sure to show work, report answers to the correct number of significant figures and
use the proper units.
1) 13 pts A metal forms the salt MCl3. Electrolysis of the molten salt with a current
of 0.700 A for 6.63 h produced 3.00 g of the metal. What is the molar mass
of the metal?
Determine the moles of electrons
π =πΌπ‘
πΉ
π‘ = 6.63β (60 πππ
1 β) (
60 π
1 πππ) = 23,900 π
π =πΌπ‘
πΉ=
(0.700 π΄)(23,900 π )
96,485 πΆπππ
= 0.173 πππ
Because M forms the compound MCl3 the ion that M forms is M3+
M3+ + 3e- M
Determine the moles of M
0.173 πππ πβ (1 πππ π
3 πππ πβ) = 0.0577 πππ π
Determine molar mass M
3.00 π
0.0577 πππ= 52.0 π
πππ
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2) 17 pts Consider the process
A(l, 75Β°C) A(g, 155Β°C)
which is carried out at constant pressure. The total ΞS for this process is
known to be 75.0 π½
πΎβπππ. For A(l) and A(g), CP values are 75.0 π½
πΎβπππ, and
29.0 π½
πΎβπππ, respectively, and are not dependent on temperature. Calculate
ΞHvap for A(l) at 125Β°C (its boiling point).
Break this process into 3 steps
A(l) (75Β°C to 125Β°C)A(l) to Al(g) 125Β°C A(g) (125Β°C to 155Β°C)
Step 1:
βπ = πΆπππ (π2
π1) = (75.0 π½
πππβπΎ)ππ (
398πΎ
348πΎ) = 10.1 π½
πΎβπππ
Step 2:
Phase change, therefore, T is constant. The problem indicates that
P is also constant.
βπ =π
π=
βπ»π£ππ
π=
βπ»π£ππ
398πΎ
Step 3:
βπ = πΆπππ (π2
π1) = (29.0
π½
πππβπΎ) ππ (
428πΎ
398πΎ) = 2.11
π½
πΎβπππ
βππ‘ππ‘ = 10.1π½
πππβπΎ+
βπ»π£ππ
398 πΎ+ 10.1
π½
πΎβπππ= 75.0
π½
πΎβπππ
βπ»π£ππ = 2.50 Γ 104 π½
πππ= 25.0
ππ½
πππ
8
3) Calculate the number of moles of HCl(g) that must be added to 1.0 L of
1.0 M NaC2H3O2 to produce a solution buffered at each pH.
3a) 6 pts pH = pKa
Na+, C2H3O2-, H
+, and Cl-
Yes a reaction will go to completion
H+ + C2H3O2- HC2H3O2
H+ C2H3O2- HC3H5O2
Initial
(mol) x 1.0 0
Final (mol) 0 1.0-x x
Major Species: C2H3O2- , HC2H3O2, Na+, and Cl-
Buffer
When the pH = pKa [HC2H3O2] = [C2H3O2-]
[π΄β]
[π»π΄]= 1 =
1.0 πππβπ₯
ππ₯
π
π₯ = 0.5 πππ 0.5 moles of HCl is needed
3b) 6 pts pH = 4.20
Still have a buffer
ππ» = ππΎπ + πππ ([π΄β]
[π»π΄])
ππΎπ = βπππ(1.8 Γ 10β5) = 4.74
4.20 = 4.74 + πππ (
1.0 πππ β π₯ππ₯π
)
0.29 =1.0 πππ β π₯
π₯
π₯ = 0.78 π 0.78 moles of HCl is needed.
3c) 6 pts pH = 5.00
Still have a buffer
ππ» = ππΎπ + πππ ([π΄β]
[π»π΄])
5.00 = 4.74 + πππ (
1.0 πππ β π₯ππ₯π
)
1.8 =1.0 πππ β π₯
π₯
π₯ = 0.36 π
0.36 moles of HCl is needed.
9
4a) 10 pts A technician carries out the reaction 2SO2(g) + O2(g) 2SO3(g) at 25ΛC
and 1.00 atm in a constant-pressure cylinder fitted with a piston. Initially,
0.0300 mol SO2 and 0.0300 mol O2 are present in the cylinder. The
technician then adds a catalyst to initiate the reaction. How much work
takes place, and is it done by the system or on the system. Assume that the
reaction goes to completion and the temperature of the system is constant.
π€ = βπβπ Determine the change in volume
Determine initial volume
ππ = ππ π
π =ππ π
π=
(0.0600 πππ)(0.08206 πΏβππ‘ππππβπΎ)(298πΎ)
1.00 ππ‘π= 1.47 πΏ
Determine the limiting reagent and final moles of substances
Total moles of gas in the container after the reaction comes to completion
0.0150 πππ + 0.0300 πππ = 0.0450 πππ Determine final volume
π =ππ π
π=
(0.0450 πππ)(0.08206 πΏβππ‘ππππβπΎ
)(298πΎ)
1.00 ππ‘π= 1.10 πΏ
Calculate work
βπ = 1.10πΏ β 1.47πΏ = β0.37πΏ
π = 1.00ππ‘π (101325 ππ
1 ππ‘π) = 101325 ππ
π€ = βπβπ = β(1.00 ππ‘π)(β0.37 πΏ) = 0.37 πΏ β ππ‘π
0.37 πΏ β ππ‘π (101.325 π½
1 πΏβππ‘π) = 37π½
Work is done on the system
4b) 6 pts What is the change in enthalpy for the reaction in J?
βπ»ππ₯πΒ° = β βπ»π
Β°(ππππ) β β βπ»πΒ°(πππππ‘)
βπ»ππ₯πΒ° = 2βπ»π
Β°(ππ3) β 2βπ»πΒ°(ππ2) β βπ»π
Β°(π2)
βπ»ππ₯πΒ° = 2 (β396
ππ½
πππ) β 2 (β297
ππ½
πππ) β (0
ππ½
πππ) = β198
ππ½
πππ
This is the amount of heat released when 1 mole of O2 reacts with 2 mol of SO2
Calculate ΞH for 0.015 mol O2
0.15 πππ π2 (β198 ππ½
1 πππ π2
) = β3.0 ππ½ = β3.0 Γ 104π½
4c) 4 pts What is the change in internal energy for the reaction in J? Since the reaction takes place at constant pressure ΞH = q
βπΈ = π + π€ = β3.0 Γ 104 π½ + 37 π½ = β3.0 Γ 104 π½
SO2 (L.R) O2 SO3
I 0.0300
mol
0.0300
mol
0
F 0 mol 0.0150
mol
0.0300
mol
10
Multiple Choice Questions Each of the following multiple choice questions are worth 5 points. Your
answers need to be filled in on the Scantron provided. Note: Your
Scantrons will not be returned to you, therefore, for your records, you may
want to mark your answers on this sheet. On the Scantron you need to fill
in your perm number, test version, and name. Failure to do any of these
things will result in the loss of 1 point. Your perm number is placed and
bubbled in under the βID numberβ. Do not skip boxes or put in a hyphen.
In addition, leave bubbles blank under any unused boxes. The version
number (A) is bubbled in under the βtest form.β
1.The pH of a 0.10 M solution of a weak base is 9.82. What is the Kb for this base?
A) 6.6 10-4
B) 4.3 10-8
C) 2.0 10-5
D) 2.1 10-4
E) None of the above
2. For the vaporization of a liquid at a given pressure,
A) G is negative at all temperatures.
B) G is negative at low temperatures but positive at high temperatures (and zero at
some temperature).
C) G is positive at all temperatures.
D) G is positive at low temperatures but negative at high temperatures (and zero at
some temperature).
3. Predict which of the following reactions has a negative entropy change.
I. CH4(g) + 2O2(g) CO2(g) + 2H2O(l)
II. NH3(g) + HCl(g) NH4Cl(s)
III. 2KClO4(s) 2KClO3(s) + O2(g)
A) II
B) I and II
C) I
D) III
E) II and III
11
4.Arrange the following 0.10 M solutions from lowest to highest pH: NaF, NaC2H3O2,
C5H5NHCl, KOH, HCN. (Ka for HCN is 6.2 10β10; Ka for HF is 7.2 10β4; Ka for HC2H3O2
is 1.8 10β5; and Kb for C5H5N is 1.7 10β9)
A) HCN, C5H5NHCl, NaF, NaC2H3O2, KOH
B) C5H5NHCl, HCN, NaF, NaC2H3O2, KOH
C) NaF, NaC2H3O2, HCN, C5H5NHCl, KOH
D) KOH, NaC2H3O2, NaF, HCN, C5H5NHCl
E) None of the above
5. For a reaction in a voltaic cell, both HΒ° and SΒ° are positive. Which of the following
statements is true?
A) GΒ° > 0 for all temperatures.
B) EΒ°cell will increase with an increase in temperature.
C) EΒ°cell will decrease with an increase in temperature.
D) EΒ°cell will not change when the temperature increases.
E) None of the above statements is true.
Use the following to answer questions 4-5:
Consider the equation 2A(g) 2B(g) + C(g). At a particular temperature, K = 1.6 104.
6. Raising the pressure by lowering the volume of the container will
A) cannot be determined
B) none of the these
C) cause [B] to increase.
D) have no effect.
E) cause [A] to increase.
7. If you mixed 5.0 mol B, 0.10 mol C, and 0.0010 mol A in a 1-L container, in which direction
would the reaction initially proceed?
A) The above mixture is the equilibrium mixture.
B) To the left.
C) We cannot tell from the information given.
D) To the right.
8. A concentration cell is constructed using two Ni electrodes with Ni2+ concentrations of 1.26 M
(cathode) and 4.95 10β4 M (anode) in the two half-cells. The reduction potential of Ni2+ is β
0.23 V. Calculate the potential of the cell at 25Β°C.
A) +0.101 V
B) +0.331 V
C) β0.201 V
D) β0.0287 V
E) β0.232 V
12
9. At 25Β°C, the following heats of reaction are known:
2C2H2 + 5O2 4CO2 + 2H2O H = β2600.0 kJ
C + O2 CO2 H = β394 kJ
2H2 + O2 2H2O H = β572 kJ
At the same temperature, calculate ΞH for the following reaction:
2C + H2 C2H2 H = ?
10. Of the following five ions or molecules, which is the strongest reducing agent?
A) Cl2
B) Fe2+
C) Cr2+
D) F-
E) H2
11. Ammonium metavanadate reacts with sulfur dioxide in acidic solution as follows (hydrogen
ions and H2O omitted):
xVO3β + ySO2 xVO2+ + ySO4
2β
The ratio x : y is
A) 2 : 1
B) 1 : 1
C) 1 : 3
D) 1 : 2
E) None of the Above
12. The following reaction has a GΒ° value of 42.6 kJ/mol at 25Β°C.
HB(aq) + H2O(l) H3O+(aq) + Bβ(aq)
Calculate Ka for the acid HB.
A) 3.41 10β8
B) 14.0
C) 42,600
D) β17.2
E) 1.63
B, D, B, B, B, E, B, A, B, C, A, A
A) 2422 kJ
B) 226 kJ
C) -2422kJ
D) -226 kJ
E) none of these