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Chemistry 111 Sect. 3
Exam #1
October 5, 2000
Name: _____________________ KEY _____________________
Student #: ______________________________________________
Please put your name and student number on all pages in case they become separated. The
last page of the exam has some useful information. The exam lasts 60 minutes. There are a
total of 100 points.
MULTIPLE CHOICE QUESTIONS
Place an X in the box corresponding to the correct answer.
1. (5 pts) The name of the compound Na2SO4 is:
o
o
oX
o
o
nitrogen sulfide
nitrogen sulfite
sodium sulfate
sodium sulfite
nickel sulfide
2. (5 pts) An ion with the symbol
138
2+
56 Ba
is correctly described by which of the following?
Numbers of protons, neutrons, electrons:
o 56, 138, 56
o
oX
o
o
69, 69, 54
56, 82, 54
56, 82, 56
56, 82, 58
The top number is mass number A; the bottom is atomic # Z. Z is
# of protons in the nucleus, A is the sum of # of protons plus # of
neutrons. In the neutral (uncharged) atom, the # of electrons
equals the # of protons. For this particular ion, Z=56, A=138, and
there are two fewer electrons than protons. Thus, # prot = 56, #
neutr = 138 - 56 = 82, and # electr = 56 - 2 = 54.
3. (5 pts) You measure the three sides of a cube as 4.021 cm, 11.6 cm and 12.5 mm. The
volume of a cube is length × width × height. What is the correct number of significant
figures that you should use to report the measured volume of the cube?
o
o
oX
o
o
1
2
3
The # of SF retained for a product of two numbers is the smaller #
of SF of the two numbers. The smallest # of SF for the three
numbers here is 3, so that is the correct # of SF in the final
answer, which is 5.83 cm3.
4
can’t tell
Page 1
Chemistry 111 Sect. 3
Exam #1
October 5, 2000
Name: _____________________ KEY _____________________
Student #: ______________________________________________
4. (5 pts) Succinic acid has an empirical formula of C2H3O2 and a molecular weight of 118
grams/mol. What is the molecular formula of succinic acid?
o
o
oX
o
o
C8H12O8
C2H3O2
C4H6O4
C6H12O6
The empirical (simplest) formula, if that is also the true molecular
formula would have a molecular mass of 59 g/mol (2 carbon + 3
hydrogen + 2 oxygen = 2•12 g/mol + 3•1 g/mol + 2•16 g/mol).
The true succinic acid molecular mass is twice that, so there must
be twice as many copies of C, H and O in the correct formula.
C3H6O3
5. (5 pts) A 1.000 g sample of metal X combines with 0.2200 g of oxygen. Calculate the
atomic mass of X in amu if the formula of the compound formed is XO2.
o
o
o
o
oX
0.78
9.1
72.7
78
145.5
Start by determining the # of moles of oxygen that is combined with
the metal: # mol of O = (0.2200 g)/(16 g/mol) = 0.01375 mol. Note
that there are half as many moles of metal X as there are of oxygen
in the compound, so # mol of X = (0.01375 mol)/2 = 0.006875 mol.
Since 1 g of X makes up this 0.006875 mol, the atomic mass of X is
(1.000 g)/(0.006875 mol) = 145.5 g/mol. The definition of amu,
which is based on 12C, is that one 12C has a mass of exactly 12 amu.
Molar mass is also based on 12C, which is defined to have a mass of
exactly 12 g/mol. So the numerical value in g/mol is the same
numerical value in amu for the elements.
6. (5 pts) When the following equation is balanced with the smallest whole number
coefficients possible, the coefficient of KNO3 is:
___K3PO4 + ___Ca(NO3)2 → ___KNO3 + ___ Ca3(PO4)2
o
o
o
o
oX
2
3
4
5
It takes 3 Ca2+ for every 2 PO43- to make calcium phosphate. That
means we need three calcium nitrates and two potassium
phosphates, which leaves us with six potassium nitrates in the
products.
6
7. (5 pts) What is the correct net ionic equation for the reaction (if a reaction occurs) between
sulfuric acid and sodium hydroxide?
Page 2
Chemistry 111 Sect. 3
Exam #1
October 5, 2000
o
o
o
oX
o
Name: _____________________ KEY _____________________
Student #: ______________________________________________
H2SO4(aq) + 2 NaOH(aq) → Na2SO4(aq) + 2 H2O(l )
H2SO4(aq) + 2 OH-(aq) → SO42-(aq) + 2 H2O(l)
Na+(aq) + SO42-(aq) → Na2(SO4)(aq)
H+(aq) + OH-(aq) → H2O(l)
No reaction
Sulfuric acid and sodium hydroxide
are strong electrolytes. They
completely dissociate in water. The
two ions that will react in the NIE
are H+ and OH-. The other ions are
soluble as Na2SO4 is a strong
electrolyte.
8. (5 pts) The oxidation number of Mn in K2MnO4 is:
o
o
o
oX
o
First note that permanganate has a charge of -2. Now apply the
rules of precedence for oxidation numbers. O is always -2 unless
it is bound to F or another O, so the total charge associated with
the 4 Os is -8. Since the permanganate carries a net -2 charge,
then Mn in this polyatomic ion must have an oxidation # of (-2) (-8) = +6.
-2
0
+2
+6
+7
MULTIPLE MULTIPLE CHOICE QUESTIONS
Place an X in the boxes corresponding to the correct answers. Any number of answers may
be correct, including none of them.
9. (10 pts) In lecture we discussed the reaction between the insoluble compound calcium
hydroxide and sulfuric acid. In the demonstration, the calcium hydroxide dissolved when
sulfuric acid was added to it. In this reaction, what are the spectator ions?
Ca(OH)2(s) + H2SO4(aq) → CaSO4(aq) + H2O(l)
o
o
o
oX
o
Ca2+(aq)
-
OH (aq )
H+(aq)
SO42+(aq )
CaSO4(aq)
Calcium hydroxide is a weak base. Sulfuric acid is a strong acid.
Thus, the species that are present as reactants are Ca(OH)2, H+
and SO42-. Because water is formed (this is an acid-base
reaction), the two species that react are Ca(OH)2 and H+:
Ca(OH)2 + H+ → Ca2+ + H2O
Sulfate does not participate; it is the spectator. Ca2+ is not a
spectator because it is only on the product side of the reaction.
10. (10 pts) You are asked to convert the number of grams of copper to numbers of atoms of
copper. During the calculation, what units “cancel out”.
Page 3
Chemistry 111 Sect. 3
Name: _____________________ KEY _____________________
Exam #1
Student #: ______________________________________________
October 5, 2000
To convert grams to atoms, we need to first convert grams to
o mL
moles and then use Avogadro’s number to convert moles to
oX g
number of atoms:
o atoms
[g]/[g/mol]•[# atoms/mol]
oX mol
o
percent
The units that have “canceled” are g and mol.
11. (10 pts) Which of the following are TRUE?
o
oX
oX
o
o
¼ mole of N2 contains 3.0 × 1023 molecules 1 mole has 6×1023 molecules; ¼ has 1.5×1023
the density of hydrogen gas at room temperature and atmospheric pressure is less than
that of air recall the balloons in the first class
all redox reactions include reducing agents and oxidizing agents Always true
the temperature at which water boils is close to 460 K 100 °C = (100 + 273) K = 373 K
all ionic compounds dissolve readily in water precipitating salts don’t dissolve readily
SHORT ANSWER QUESTIONS
Answer in the spaces indicated.
12. (10 pts) When the following skeletal equation is balanced under acidic conditions, what
are the coefficients of the species shown?
2+
_____
1 Cd(s) + _____
1 ClO4 (aq) → _____
1 Cd (aq) + _____
1 ClO3 (aq )
Ox. ½ react: Cd → Cd2+ + 2 eRed. ½ react: 2 e- + 2 H+ + ClO4- → ClO3- + H2O
Cd
Which species is the reducing agent? _______
Two electrons in each reaction means that 1 of the ox. ½
½ reactions add to make the
ClO4 reactions and +1 of the red.
Which species is the oxidizing agent? _______
net: Cd + 2 H + ClO4 → Cd2+ + ClO3- + H2O
-
Cd provides the electrons, it is the reducing agent. Cl in
ClO4 changes ox. # from +7 to +5; it is the oxidizing
agent in the species perchlorate.
13. (10 pts) Fill in the blanks in the table (one column per element).
Symbol
58
Ni
20
Ne
___________
Page 4
The atomic mass A in the
55
Mn
___________
superscript is the sum of #
of prot. plus neutr. The #
of e- in the neutral atom is
equal to the # of protons.
Chemistry 111 Sect. 3
Exam #1
October 5, 2000
Number of protons
Number of neutrons
Number of electrons
in the neutral atom
Name of element
Name: _____________________ KEY _____________________
Student #: ______________________________________________
___________
28
30
___________
10
10
___________
25
30
28
___________
nickel
___________
10
___________
neon
___________
25
manganese
___________
14. (10 pts) Do you agree with the following statement? Please try to answer in one or two
sentences at most.
“The number of grams in one mole of 20Ne is exactly 20.”
No. The mole is defined based on exactly 12 g of 12C. All other elements and isotopes do
not have exact integer atomic masses.
Page 5
Chemistry 111 Sect. 3
Exam #1
October 5, 2000
Name: _____________________ KEY _____________________
Student #: ______________________________________________
Useful information
NA = 6.02 × 10 23 molecules/mol
d = m/V
PERIODIC TABLE OF THE ELEMENTS
IA
IIA
IIIB
IVB
VB
VIB
VIIB
VIIIB
IB
IIB
IIIA
IVA
VA
VIA
VIIA VIIIA
1
2
H
He
1.008
4.003
3
4
5
Li
Be
B
C
N
O
F
Ne
6.939
9.012
10.81
12.01
14.01
16.00
19.00
20.18
11
12
13
14
15
16
17
18
Na
Mg
Al
Si
P
S
Cl
Ar
22.99
24.31
26.98
28.09
30.97
32.07
35.45
39.95
19
20
21
22
K
Ca
Sc
Ti
39.10
40.08
44.96
37
38
39
Rb
Sr
85.47
23
6
7
8
9
10
24
25
26
27
28
29
30
31
32
33
34
35
36
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
47.90
50.94
52.00
54.94
55.85
58.93
58.71
63.55
65.39
69.72
72.61
74.92
78.96
79.90
83.80
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
87.62
88.91
91.22
92.91
95.94
(99)
101.1
102.9
106.4
107.9
112.4
114.8
118.7
121.8
127.6
126.9
131.3
55
56
57
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
Cs
Ba
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
132.9
137.3
138.9
178.5
181.0
183.8
186.2
190.2
192.2
195.1
197.0
200.6
204.4
207.2
209.0
(209)
(210)
(222)
87
88
89
Fr
Ra
Ac
(223)
226.0
227.0
Page 6
I
Xe