Chem 127, Exam III
November 29, 2001
Name
I. (40 points)
A. (10 points) Write the symbol of the element
1. of lowest atomic number which could have the following excited state:
1s2 2s2 2p5 3s1
2. in Group 14 that has the smallest atomic radius.
3. in Group 15 that is the most electronegative.
4. in Period 2 that has the highest first ionization energy.
5. that is a transition metal and in Period 4 that has no unpaired electrons.
B. (5 points) Write the electron configuration and orbital diagram for cobalt in its ground
state. Answer the following questions and write your answers on the blanks provided.
1. What is the highest value for n in the electron configuration?
2. What is the highest numerical value for in the electron configuration?
3. How many electrons with = 1 does cobalt in its ground state have?
4. How many unpaired electrons are there?
5. How many electrons with = 2 does Co2+ have?
C. (8 points) The order of sublevels is s, p, d, f, g, h, etc. For the g sublevel:
1. What is the quantum number designation?
2. What is the lowest value of n for which the g sublevel is possible?
3. What is the maximum number of orbitals in this sublevel?
4. What is the total number of electrons in the g sublevel?
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Chem 127, Exam III
November 29, 2001
Name
D. (10 points) Write your answers to the following questions on the blanks provided.
1. What is the minimum quantum number for n in the following set of quantum
numbers?
n = ?; = 2; m = 0 ; ms = + 1/2
2. What is the quantum number for in the following set of quantum
numbers?
n = 2; = ?; m = -1 ; ms = - 1/2
3. Which of the following is a possible set of quantum numbers for a 3d
electron? (Write the number of the set on the blank provided.)
(1)
n = 3; = 3; m = -2 ; ms = + 1/2
(2)
n = 3; = 1; m = 0 ; ms = - 1/2
(3)
n = 3; = 2; m = -2 ; ms = + 1/2
(4)
n = 3; = 1; m = 1 ; ms = + 1/2
4. Compare the energy required for an n = 1 to n = 2 transition (E1 ) to the
energy required of an n = 2 to n = 3 transition (E2 ). (Given below are 4
choices. Write the number of your choice for the correct answer in the
blank provided.)
(1)
E1 = E2
(2)
E1 > E2
(3)
E1 < E2
(4)
not enough information to tell
5. When an electron makes a transition from n = 4 to n = 2, energy is
.
(Given below are 3 choices. Write the number of your choice for the correct
answer in the blank provided.)
(1)
given off
(2)
absorbed
(3)
not a factor
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Chem 127, Exam III
November 29, 2001
Name
E. (7 points) These 2 problems are not related to each other. There will be no partial
credit given for these problems. You must show all work in order to receive credit.
1. What is the wavelength (in m) of a microwave radiation associated with a frequency of 1.00 x 1010 s−1 ?
2. What is the energy difference (in J) for a transition from n = 1 to n = 3 in the
hydrogen atom?
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Chem 127, Exam III
November 29, 2001
Name
II. (40 points) Draw the Lweis structures of the molecules and answer the following
questions.
A. (12 points) SeO2 with Se as the central atom.
1. What is the total number of valence electrons for the molecule?
2. How many unshared pairs (lone pairs) are there around the central atom?
3. What is the geometry of the molecule?
4. What is the O–Se–O bond angle?
5. Is the molecule polar?
6. What is the hybridization of the central atom?
B. (12 points) N2 O4 has the following skeleton:
O–N–N–O
|
|
O O
1. What is the formal charge on N?
2. What is the lowest formal charge for an oxygen atom in this molecule?
3. How many pi bonds are in this molecule?
4. How many sigma bonds are in this molecule?
5. How many different resonance structures can you write for this molecule?
6. What is the hybridization of the nitrogen atoms?
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Chem 127, Exam III
November 29, 2001
Name
C. (12 points) Given below are 2 possible Lewis structures for POCl3 . Answer the questions about each structure on the blanks provided.
..
:O:
.. | ..
:Cl
.. – P – Cl:
..
|
:Cl:
..
:O:
.. ..
:Cl
.. – P – Cl:
..
|
:Cl:
..
1. formal charge of
phosphorus
2. formal charge of
oxygen
3. molecular geometry
4. number of π bonds
5. number of σ bonds
6. Is the molecule
polar?
D. (4 points)
1. Given the following four bonds, which bond is the most polar?
C–C
C–N
C–O
C–F
2. the structure of the carbonate ion is given below. How many resonance
structures can be drawn for this ion counting the given structure as the
first one?
..
..
:O
.. – C – O:
..
:O:
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Chem 127, Exam III
November 29, 2001
Name
III. (40 points)
A. (10 points) These 2 questions are not related.
1. Given
2 Fe2 O3 (s) → 4 Fe (s) + 3 O2 (g)
∆H = 1648.4 kJ
What is the heat of formation of iron (III) oxide?
2. Consider the following reaction
C2 H4 (g) + 3 O2 (g) → 2 CO2 (g) + 2 H2 O ()
∆H = -1421.1 kJ
Given the following heats of formation:
∆H◦f CO2 (g) = -393.5 kJ/mol
∆H◦f H2 O () = -285.9 kJ/mol
What is the heat of formation of C2 H4 (g)?
B. (6 points) Determine whether the statements given below are true or false. Write your
answers in the blanks provided. If the statement is true, write Y. If the statement is
false, write N. If more information is required write MI. Do not use T or F.
1. A reaction is carried out in a test tube. After the reaction is complete, the test
tube feels cold. This means that qsurr < 0.
2. qreaction = – qcalorimeter
3. The enthalpy of vaporization (∆ Hvap ) is numerically equal but opposite in sign
to the enthalpy of fusion (∆ Hfus ).
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Chem 127, Exam III
November 29, 2001
Name
D. (12 points) Calcium carbide is formed according to the following equation.
CaO (s) + 3 C (s) → CO (g) + CaC2 (s)
When one mole of CaC2 is formed, 464.8 kJ of heat are absorbed.
1. Is the reaction endothermic or exothermic?
2. What is ∆H when 10.0 g of CaC2 is formed?
Write a plus (+) or minus (–) sign before your
numerical answer.
3. How many grams of carbon are used up when 30.0 kJ of heat are absorbed?
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Chem 127, Exam III
November 29, 2001
Name
E. (12 points) When 2.54 g of NH4 NO3 (M = 80.0 g/mol) dissolves in 72.0 g of water in
a coffee-cup calorimeter, the temperature drops from 19.54◦ C to 16.58◦ C.
1. Is the dissolving of NH4 NO3 endothermic or exothermic?
2. Based on your answer in (1), which has the greater enthalpy: 1 mol of NH4 NO3 (s)
or 1 mol of NH+4 (aq) and 1 mole of NO−
3 (aq)? (i.e. the reactants or the products)
3. What is q when 2.54 g of NH4 NO3 are dissolved?
4. What is q in kJ when one mol NH4 NO3 dissolves?
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Chem 127, Exam III
November 29, 2001
Name
IV. (30 points)
A. (12 points) Consider the following balanced redox reaction. The reaction is the basis
for the permanganate titration used to determine the % Fe in a sample.
2+
2+
3+
8 H+ (aq) + MnO−
4 (aq) + 5 Fe (aq) → Mn (aq) + 5 Fe (aq) + 4 H2 O
2.1 g of an unknown iron salt are dissolved in water, acidified with H3 PO4 and titrated
with a standardized KMnO4 solution. A persistent pink color is observed after 7.8 mL
of a 0.250 M KMnO4 solution are used.
1. What is the color of a solution of potassium permanganate?
2. How many moles of MnO−
4 are used in the titration?
3. How many moles of Fe are present in the sample?
4. What is the mass percent of Fe in the sample?
B. (6 points) It is determined that the halogens rank as oxidizing agents in the following
manner: (weakest) I2 , Br2 , Cl2 (strongest). Write R if a reaction occurs, NR if no
reaction occurs, and MI if more information is required.
1. Br2 solution + Br− solution
2. Br2 solution + Cl− solution
3. Br2 solution + I− solution
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Chem 127, Exam III
November 29, 2001
Name
C. (12 points) In experiment 8 you determined the molar mass of a volatile liquid by
vaporizing the liquid in a flask of known volume. You condensed the liquid and then
determined the mass of the condensed liquid. Below is a copy of your Data and
Calculation sheet. Fill in the blanks. Show your work on the right margin.
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Chem 127, Exam III
November 29, 2001
Name
BONUS (15 points) All or nothing.
The bonus should be done only after you have completed the main part of this exam and
checked your work for errors. The time allotted for this exam does not include time for the
bonus. (SHOW ALL WORK! Lucky guesses will not be considered.)
You are a waiter in a restaurant. You are asked for cold water at 5.0◦ C. You have a jug
of warm water at 30.0◦ C and a bucket of ice at 0◦ C. You serve water in glasses with a
volume of 400.0 mL which you are required to fill to the brim. How many g of ice do you
need to satisfy the customer? (Remember: There is no ice present at a temperature of
5◦ C.)
You may assume the following:
density of both water and ice is 1.0 g/mL
specific heat of water = 4.18 J/g◦ C
∆Hfus of ice = 0.33 kJ/g
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