Download 2011 Spring 1 key

CHEMISTRY 1A SPRING 2011
EXAM 1 KEY
CHAPTERS 1-4
You might find the following useful.
Electronegativities
H
2.2
Li
0.98
Be
1.57
B
2.04
C
2.55
N
3.04
O
3.44
F
3.98
Na
0.93
Mg
1.31
Al
1.61
Si
1.9
P
2.19
S
2.58
Cl
3.16
K
0.82
Ca
1.0
Ga
1.81
Ge
2.01
As
2.18
Se
2.55
Br
2.96
Rb
0.82
Sr
0.95
In
1.78
Sn
1.96
Sb
2.05
Te
2.1
I
2.66
Cs
0.79
Ba
0.89
Tl
2.33
Pb
2.02
Bi
2.0
Po
2.2
Xe
2.6
Densities
Substance
Density
g/mL
Substance
Density
g/mL
battery acid
1.29
magnesium, Mg
1.74
benzene, C6H6
0.879
nitrobenzene, C6H5NO2
1.20
blood (whole)
1.05
oleic acid
0.895
bromine, Br2
3.12
sea water
1.025
copper, Cu
8.94
sodium chloride, NaCl
2.16
chromium(III) oxide, Cr2O3
5.2
sulfuric acid (concentrated)
1.84
gasoline
0.7025
triethylene glycol
1.12
Gold, Au
19.0
trinitrotoluene, TNT
1.654
Iron, Fe
7.87
water at 20 C
0.997
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Answer the following by writing the word, words, letter, letters or number in each blank
that best completes each sentence. (1 point each blank)
1. Isotopes are atoms that have the same number of protons but different numbers of
neutrons. They have the same atomic number but different mass numbers.
2. Electronegativity is a measure of the electron-attracting ability of an atom in a chemical
bond.
3. A(n) oxyanion is a polyatomic ions with the general formula HaXbOcd-. (The a can be 0.)
4. Isomers are compounds that have the same molecular formula but different molecular
structures.
5. Molar mass is the mass in grams of one mole of substance.
6. A(n) formula unit is a group represented by a substance’s chemical formula, that is, a group
containing the kinds and numbers of atoms or ions listed in the chemical formula. It is a
general term that can be used in reference to elements, molecular compounds, or ionic
compounds.
7. The accepted SI unit for amount of substance is the mole, which has an abbreviation of
mol.
8. The accepted SI base unit for energy is the joule with an abbreviation of J.
9. The chemical symbol for cobalt is Co, for potassium is K, and for silver is Ag.
10. The names of elements that correspond to the following symbols are sodium for Na,
cadmium for Cd, and manganese for Mn.
11. The name of the group on the periodic table in which barium, Ba, is found is alkaline earth
metal. Its group number is 2 or 2A. Barium is in the 6th period. It is a representative (or
main-group) element (representative element, transition metal or inner transition metal).
12. The N-F bond is a(n) polar covalent (ionic, polar covalent, or nonpolar covalent) bond.
13. In the C-O polar covalent bond, the O atom has the partial negative charge.
14. Draw a reasonable Lewis structure for COBr2. (4 points)
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15. Identify each of the following as a binary covalent compound, a binary ionic compound, a
binary acid, an ionic compound with a polyatomic ion, an oxyacid, an alcohol, or a sugar.
Write name for each. (8 points)
Chemical
Formula
Type of Substance
Name
HNO2
oxyacid
nitrous acid
Co(ClO)3
ionic with polyatomic ion
cobalt(III) hypochlorite
C2H6
binary covalent (hydrocarbon)
ethane
Ca(HCO3)2
ionic with polyatomic ion
calcium hydrogen carbonate
16. Identify each of the following as a binary covalent compound, a binary ionic compound, a
binary acid, an ionic compound with a polyatomic ion, an oxyacid, an alcohol, or a sugar.
Write formula for each. (8 points)
Chemical Formula
Type of Substance
Formula
ferrous sulfate
ionic with polyatomic ion
FeSO4
sulfur hexafluoride
binary covalent
SF6
hydrochloric acid
binary acid
HCl(aq)
ethanol
alcohol
C2H5OH
or
CH3CH2OH
17. Complete the following table by (1) writing the name for the type of particle viewed as
forming the structure of a solid, liquid, or gas of each of the following substances and (2)
writing the name of the type of attraction holding these particles in the solid and liquid
form. (9 point each box)
Substance
Particles to Visualize
Type of Attraction
CH3OH
molecules
hydrogen bonds
NH4OH
cations and anions
ionic bonds
sulfur
molecules
London forces
cadmium
cations in sea of e-
metallic bonds
neon
atoms
London forces
C6H14
molecules
London forces
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18. Draw the Lewis structure for 2,2,4-trimethylpentane. (3 points)
19. Write the name for the compound represented by the Lewis structure below. (3 points)
3-ethylheptane
For each of the following numerical problems, be sure to show your work.
NOTE: Remember that there is part credit for each problem. Even if you cannot do all of
a problem, be sure to set up as much of it as you can.
20. Chromium metal is used in metal alloys and as a surface plating on other metals to
minimize corrosion. It can be obtained by reducing the chromium(III) in chromium(III)
oxide, Cr2O3, to the uncharged metal with finely divided aluminum. What is the maximum
mass, in pounds, of chromium that can be made from a sample of Cr2O3 with a volume of
2.00 m3 of Cr2O3? (8 points)
3
 102 cm   1 mL   5.2 g Cr2O3   1 mol Cr2 O3 
? lb Cr = 2.00 m Cr2O3 


 
3 
 1 m   1 cm   1 mL Cr2O3   151.990 g Cr2O3 
3
 2 mol Cr   51.9961 g Cr   1 lb 
4
 = 1.6 ×10 lb Cr



 1 mol Cr2 O3   1 mol Cr   453.6 g 
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21. Potassium permanganate, KMnO4, a common oxidizing agent, is made from various ores
that contain manganese(IV) oxide, MnO2. The following equation shows the net reaction
for one process that forms potassium permanganate.
2MnO2 + 2KOH + O2  2KMnO4 + H2
a. What is the maximum mass, in kilograms, of KMnO4 that can be made from the
reaction of 835.6 g of MnO2 with 585 g of KOH and excess oxygen gas? (8 points)
 2  158.0339 g KMnO4   1 kg 
? kg KMnO4 = 835.6 g MnO2 
  3  = 1.519 kg KMnO4
 2  86.9368 g MnO2   10 g 
 2  158.0339 g KMnO4   1 kg 
? kg KMnO4 = 585 g KOH 
  3  = 1.65 kg KMnO4

2
56.1056
g
KOH

  10 g 
b. Explain why the oxygen gas is in excess. (6 points)
Because air contains oxygen gas, the oxygen is less expensive than the other reactants.
It’s especially important that the manganese reactant is used most efficiently, so it is
limiting. Oxygen is very safe, so we don’t mind having some left over. Because the O2 is
a gas, it is easily separated from the solid product, KMnO4.
c. If 1.18 kg of KMnO4 are isolated from the product mixture of the reaction of 835.6 g of
MnO2 with 585 g of KOH and excess oxygen gas, what is the percent yield? (3 points)
1.18 kg KMnO4
actual yield
% yield =
 100 =
 100 = 77.7% yield
theoretical yield
1.519 kg KMnO4
d. Explain why the actual yield in a chemical reaction such as this one is less than the
theoretical yield. (6 points)
(1) Many chemical reactions are significantly reversible. Because there is a constant
conversion of reactants to products and products to reactants, the reaction never
proceeds completely to products. (2) It is common, especially in reactions involving
organic compounds, to have side reactions. These reactions form products other than the
desired product. (3) Sometimes a reaction is so slow that it has not reached the
maximum yield by the time the product is isolated. (4) Even if 100% of the limiting
reactant proceeds to products, the product still usually needs to be separated from the
other components in the product mixture. (The other components include excess
reactants, products of side reactions, and other impurities.) This separation generally
involves some loss of product.
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Answer the following in short answer form.
22. Consider the element bromine. (8 points)
a. Describe the particles that form the basic structure of bromine.
Bromine is composed of Br2 molecules, which consist of two bromine nuclei
surrounded by a70 electron cloud. Each nucleus contains 35 protons and has a
+35 charge.
b. Is bromine a solid, liquid or gas at room temperature and pressure? liquid
c. What is the approximate percentage of the volume of a sample of bromine that is
actually occupied by particles? 70%
d. What type of attractions hold bromine particles together? London forces
e. Describe the motion of a typical bromine particle at room temperature and pressure.
Particles are constantly moving.
Attractions are strong but not strong enough to keep particles from moving
throughout the liquid.
Constant collisions that lead to changes in direction and velocity.
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