CHEM 3: Midterm Exam 2: Spring 2016
Name:________________________________________
INSTRUCTIONS: On the front AND back of the scantron, fill out your name and 7-digit student ID. Fill in
this information from left to right in the provided fields, and BUBBLE IN the values on the scantron. 1
point will be deducted from your exam grade for incomplete or erroneous filling of the scantron.
Please read each question carefully. Where the question specifies “MARK ALL THAT APPLY”, you MIGHT
need to select more than one answer on the scantron.
1.
MARK ALL THAT APPLY: Select ALL of the formulas below which are reasonable EMPIRICAL formulas.
A. C6H3(OH)3
B. (NH4)2SO4
C. CH3COCH3
D. HCCCH2CH3
E. None of the above are reasonable empirical formulas.
2.
MARK ALL THAT APPLY: Select the statements below that are TRUE.
A. If the empirical formula of a substance is C2H4O2, the molecular formula would be CH2O.
B. The molecular formula of a substance is determined from its elemental percent composition by mass.
C. For some substances, the molecular formula is equivalent to the empirical formula.
D. Two different substances having the same elemental percent composition by mass but different molecular formulas,
would still have the same empirical formula.
E. None of the above are TRUE statements.
3.
In a 100. g sample of sulfur dioxide, how many grams would be sulfur? In other words, determine the percent composition
of sulfur BY MASS in the substance sulfur dioxide.
A. 33.3 g
B. 25.0 g
C. 50.1 g
D. 66.7 g
E. None of the above
4.
Agrocelhone is a colorless liquid with a sweet smell, composed of 32.46 % carbon, 3.63 % hydrogen, and 63.91 % chlorine
by mass. It is used mainly in farming as a pesticide, specifically as a pre-planting fumigant and nematicide. Determine its
empirical formula.
A.
B.
C.
D.
E.
C1H2Cl
C2H2Cl
C3H2Cl
C3H4Cl
C3H4Cl2
5.
A substance has an empirical formula of C2H2O, with a molecular formula molar mass of 126.1 g/mol. What is the
molecular formula of this substance?
A. In this case, the empirical formula is equivalent to the molecular formula.
B. C4H4O2
C. C6H6O3
D. C3H3O3
E. Not enough information is provided: the elemental percent composition by mass must be given.
For questions # 6-11, categorize each reaction as one of the possibilities below:
A.) Synthesis
B.) Double Replacement
C.) Combustion
D.) Decomposition
E.) Single Replacement
Each reaction only fits one category, but one category may appear more than once.
6.
2 Al + 3 Br2 → 2 AlBr3
7.
3 Zn + 2 FeF3 → 3 ZnF2 + 2 Fe
8.
8 SCl6 → S8 + 24 Cl2
9.
Pb(NO3)2 + (NH4)2SO4 → PbSO4 + 2 NH4NO3
10. 2 C8H18 + 25 O2 → 16 CO2 + 18 H2O
11. U + 3 F2 → UF6
12. MARK ALL THAT APPLY: Mark any reactions below that are NOT properly balanced.
A. 2 CO(NH2)2 → 2 CO2 + 2 NH3
B. 2 C7H14O2 + 19 O2 → 14 CO2 + 14 H2O
C.
4 Al + 3 CO2 → Al4C3 + 5 O2
D. N2O4 + 2 N2H4 → 3 N2 + 4 H2O
E.
None of the above: ALL of these reactions are balanced.
13. In the equation below, what should the coefficient be on the copper atoms (Cu) when properly balanced with the smallest
whole number coefficients?
____ Cu + ____ Mn2O7 → ____ Mn + ____Cu2O
A.
B.
C.
D.
E.
2
3
7
9
14
14. If the reaction below was balanced properly with the smallest whole number coefficients, what would be the stoichiometric
coefficient in front of the diatomic oxygen (O2)?
_____ C10H20O + ____ O2 → ____ CO2 + ____ H2O
A.
B.
C.
D.
E.
29
30
58
60
None of the above are correct.
15. When properly balanced with the smallest whole number coefficients, what would be the stoichiometric coefficient in front
of the diatomic oxygen (O2)?
2 Fe2S3 + ____ O2 → 4 Fe + ____ SO3
A.
B.
C.
D.
E.
3
4
6
9
18
16. MARK ALL THAT APPLY: Using the solubility chart, select ALL of the ionic compounds below that would form INSOLUBLE
precipitates in water. Note: CrO42- = chromate.
A. BaSO4
B. Li2SO4
C. TlCl
D. K2CrO4
E. None of substances above would be insoluble in water.
17. If Al(NO3)3(aq) was mixed with Na2S(aq), what is one possible product from this double replacement reaction?
A. Al3S2
B. AlNa2
C. S(NO3)3
D. AlS
E. Al2S3
18. How many moles of iron (III) oxide would be produced starting from 30.0 moles of iron metal reacting with an excess of
oxygen gas according to the equation below?
4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)
A. 15.0 moles
B. 22.5 moles
C. 30.0 moles
D. 40.0 moles
E. 45.0 moles
19. Considering the reaction below, how many moles of elemental phosphorus (P4) would be needed to fully react with 120
moles of magnesium metal according to the equation below?
6 Mg(s) + P4(s) → 2 Mg3P2(s)
A. 10. moles
B. 20. moles
C. 60. moles
D. 120 moles
E. 720 moles
20. Considering the reaction below, how many grams of chromium (III) oxide (Cr2O3) could be generated starting from 2.00
moles of Fe2(Cr2O7)3?
Fe2(Cr2O7)3 → 3 Cr2O3 + 2 Fe + 6 O2
Molar Masses (g/mol):
759.65
151.99
32.00
A. 0.00427 g
B. 104 g
C. 912 g
D. 1520 g
E. 1820 g
21. What is the theoretical yield of phosphoric acid (in grams) assuming 10.0 grams of tetraphosphorus decoxide reacts with
an excess of water according to the reaction below?
P4O10 + 6 H2O → 4 H3PO4
Molar Masses (g/mol): 283.9 18.02
97.99
A.
B.
C.
D.
E.
0.00144 g
13.8 g
40.0 g
116 g
3920 g
22. What is the LIMITING reagent, if 12.0 moles of calcium hydroxide, 6.00 moles of tetraphosphorus decoxide, and 6.0 moles
of water are mixed together and react according to the equation below?
2 Ca(OH)2 + P4O10 + 2 H2O → 2 Ca(H2PO4)2
A.
B.
C.
D.
E.
Ca(OH)2
P4O10
H2O
Ca(H2PO4)2
Since there are equal amounts of each reactant, there is no limiting reagent in this case.
23. Syngas is produced from methane and water according to the reaction below. How many grams of methane (CH4) would be
needed to fully react with 10.0 grams of water?
CH4 + H2O → CO + 3 H2
A. 20.0 g
B. 11.3 g
C. 10.0 g
D. 8.89 g
E. Cannot determine from information provided.
24. If 20.8 g of diphosphorus pentoxide reacts with an EXCESS of phosphorus pentachloride according to the reaction below,
but 109 g of POCl3 is actually obtained as the final product, what is the percent yield?
3 PCl5 +
Molar Masses (g/mol):
A.
B.
C.
D.
E.
14.7 %
19.1 %
20.6 %
70.3 %
97.0 %
208.22
P2O5 →
141.94
5 POCl3
153.32
FREE RESPONSE: Show your work for full credit. Partial credit will be assigned.
1.
The major component of biodiesel is methyl stearate, C19H38O2. Write the balanced chemical equation for the complete
combustion of this fuel. Writing the phases for each component is not necessary.
2.
Consider the two experimental observations below:
Reaction 1: (NH4)3PO4(aq) + LiOH(aq) → Generates a PRECIPITATE.
Reaction 2: LiOH(aq) + NH4NO3(aq) → Does NOT generate a precipitate.
Based on this information, write the balanced chemical equation for the precipitation in Reaction 1. BE SURE TO INDICATE THE
PHASES OF THE PRODUCTS and specify the component that is responsible for the formation of the precipitate.
EXTRA CREDIT: (10 points) Show your work for credit; no partial credit will be awarded.
“Proton” is the name of an expendable rocket used by the Russian government to deliver payloads into
orbit and space. It is considered one of the most successful heavy boosters in the history of spaceflight.
From 1965 (when it was originally designed by the Soviets as the carrier of intercontinental ballistic
missiles) to present day (now commonly used to deliver satellites to geostationary orbit), it has been
propelled by the reaction of unsymmetrical dimethyl hydrazine (UDMH, the fuel) with dinitrogen tetroxide
(the oxidant).
1.
Write the balanced chemical equation for the reaction of dimethyl hydrazine (HCH 3NNHCH3)
with dinitrogen tetroxide to produce only nitrogen gas, water and carbon dioxide.
2.
Consider a rocket designed to hold a maximum of 4.60 x 106 Liters of the fuel / oxidant mixture shown in the reaction
above. Using this maximum volume of the rocket tanks, calculate the mass (in kg) of each reactant needed to maximize the
energy released in this reaction by minimizing the amount of each unreacted reagent left after complete conversion to
the products.
Reference Information:
Density of dinitrogen tetroxide = 1.448 g/cm3
Density of unsymmetrical dimethyl hydrazine (UDMH) = 0.791 g/cm3
Mass of N2O4 = ______________________________________________
Mass of UDMH = ______________________________________________
Percent Yield = [(Actual Yield)/(Theoretical Yield)] x 100
Density = Mass/Volume