CHM134 General Chemistry I
Exam 2, Fall 2009 – Dr. Steel
Name
SOLUTIONS
Section 1: Fill in the blanks as instructed in each question.
1. (7 points)Balance each equation by providing the appropriate coefficients. If you leave a
blank before a compound it will be interpreted as a coefficient of 1.
2 C7H14 +
21 O2 →
4 C3H5(NO3)3 →
14 CO2 +
14 H2O
12 CO2 + 6 N2 + 10 H2O +
Cu + 4 HNO3 →
Cu(NO3)2 +
O2
2 NO2 + 2 H2O
2. (6 points) Indicate whether each species is soluble or insoluble in water.
soluble
MgBr2
insoluble
Al2S3
insoluble
CoCO3
soluble
NH4PO4
soluble
FeCl2
insoluble
AgCl
3. (3 points) Indicate the oxidation number of each element in these species.
C2H6O
-2 C
+1 H
K2Cr2O7
-2 O
+1 K +6 Cr
-2 O
4. (4 points) Indicate which species is the oxidizing agent and which is the reducing agent in
the reaction below.
2 KMnO4(aq) + 16 HI(aq) → 2 KI(aq) + 2 MnI2(aq) + 5 I2(aq) + 8 H2O(l)
Mn in KMnO4
oxidizing agent
I in HI
reducing agent
5. (4 points) Indicate which species is the oxidized and which is reduced in the reaction
below.
2 Na2S2O3(aq) + Br2(aq) → Na2S4O6(aq) + 2 NaBr(aq)
S in S2O3
oxidized species
Br2
reduced species
6. (3 points) Indicate the gas that is most likely to be produced from each reaction.
H2S
Na2S(aq) + 2 HBr(aq) → ????
NH3
NaOH(aq) + NH4Br(aq)
SO2
CaSO3(aq) + H3PO4(aq) → ????
→ ????
7. (10 points) Indicate whether each reaction is a precipitation, acid-base, gas forming, or
redox reaction.
Precipitation
HBr(aq) + AgNO3(aq) → HNO3(aq) + AgBr(s)
Gas Forming
2 HCl(aq) + Na2CO3(s) → 2 NaCl(aq) + H2O(l) + CO2(g)
Redox
KI(aq) + HClO(aq) → KCl(aq) + HIO(aq)
Acid-base
LiOH(aq) + HCl(aq) → H2O(l) + LiCl(aq)
Redox
2 C2H2(g) + 5 O2(g) → 4 CO2(g) + 2 H2O(g)
Section 2: Complete each question. You must show your work in order to earn full credit.
8. (9 points) What mass of chromium reacts completely with 17.8 grams of oxygen
according to the following balanced equation?
3 O2(g) + 4 Cr(s) → 2 Cr2O3(s)
17.8
1
32.00
4
3
52.00
1
38.6
9. (12 points) Sulfuric acid will dissolve aluminum metal. How many moles of hydrogen gas
can be made from the reaction of 3.80 mol aluminum with 4.25 mol H2SO4? How many
moles of the excess reactant will be leftover?
2 Al(s) + 3 H2SO4(aq) → Al2(SO4)3(aq) + 3 H2(g)
3
2
3.80
5.70
3
4.25
3
4.25
Therefore, only 4.25 moles of H2 will be produced.
The limiting reactant is the H2SO4, thus
2
4.25
3.80
3
2.83
0.97
2.83
will be leftover.
10. (8 points) How can I prepare 100. mL of a solution of lithium carbonate (LiCO3, MW =
66.96 g/mol) in water such that it has a concentration of 2.50 M? Please be specific.
2.50
0.250
66.96
1
0.100
0.250
16.74
Thus, you should dissolve 16.74 g of LiCO3 in enough water to make exactly
100. mL of solution. This would be best done in a 100-mL volumetric flask.
11. (12 points) The balanced equation for the combustion of octane appears below. If an
average car converts octane to CO2 and water with a typical percent yield of 88.4%, how
many gallons of octane are consumed by a car that annually emits 5.20×103 kg of CO2?
1 gallon = 3.7854 L, the density of octane is 0.703 g/mL
2 C8H18(g) + 25 O2(g) → 18 H2O(s) + 16 CO2(g)
First, take into account the percent yield:
0.884
5200
5200
0.884
5882
Now find the mass of octane used:
5882
1000
1
1
44.01
2
16
1.91
114.22
1
10
2.71
0.703
2.71
1
1000
10
1
3.7854
1.91
10
10
717
12. (12 points) Fructose is a common sugar found in fruit. Elemental analysis of fructose
gave the following mass percent composition: C 40.00%, H 6.72%, O 53.28%. The molar
mass of fructose is 180.16 g/mol. Find the molecular formula of fructose.
40.00
1
12.01
3.33
3.33
1
6.72
1
1.008
6.67
3.33
2
3.33
3.33
1
53.28
1
16.00
The empirical formula is CH2O
Divide the molar mass by the empirical mass and scale up:
180.16 /
30.03 /
6
Thus, the molecular formula is C6H12O6.
13. (10 points) A sugar cube has a mass of 1.157 g of sucrose (C12H22O11). How many atoms
of oxygen are in the cube?
1.157
11
1
1
342.30
6.022
10
1
2.239
10
Bonus: Circle and identify any functional groups (other than alkanes) in the structures
below.
O
OH
carboxylic acid
alkene
O
ketone
OH
alkyne
NH2
amine
alcohol