Name _________________________
Chemistry 100
Exam 3 SOLUTION
Answer the following questions in the spaces provided. You must show your work to receive credit
for an answer. Showing your work also allows for maximum partial credit. Place your final answer,
with attention to significant figures and units, in the boxes provided.
1. Provide the chemical formula for the following compounds, and then balance the equations. (It is not
necessary to show work here) [12 pts]
a. Tin (II) nitrate and aluminum chloride yields tin (II) chloride and aluminum nitrate.
Balanced Eq:
3 Sn(NO3)2 + 2 AlCl3 → 3 SnCl2 + 2 Al(NO3)3
b. Iron (IV) sulfide + oxygen molecule yields iron (III) oxide and sulfur dioxide.
Balanced Eq:
2 FeS2 + 11/2 O2 → Fe2O3 + 4 SO2
2. Balance the following equations. (It is not necessary to show work here) [8 pts]
a.
C6H6O2 +
Balanced Eq:
b.
Balanced Eq:
O2 →
CO2 +
H2O
C6H6O2 + 13/2 O2 → 6 CO2 + 3 H2O
PCl5 +
H2O →
H3PO4 +
HCl
PCl5 + 4 H2O → H3PO4 + 5 HCl
3a. Alka Seltzer® tablets contain both citric acid, C6H8O7 (molar mass = 192.12 g/mol), and sodium
bicarbonate; when they get wet, the two react together and create the carbon dioxide that is responsible
for the “fizz”. If a typical Alka Seltzer® tablet contains 0.0250 g of citric acid, how many grams of
hydrogen does it provide to the solution? [10 pts]
0.0250 g citric acid x
1 mol citric acid
8 moles H
1.0079 g H
x
x
= 1.05 x 10-3 g H
192.12 g citric acid 1 mol citric acid
1 mol H
! This ratio comes from the formula for citirc acid: C6 H 8 O7
Amount of H: 1.05 x 10-3 g H
Exam 3
Fall 2007
Page 1 of 4
Chemistry 100
Clark College
3. Valeric acid can be extracted from the valerian plant and smells like dirty socks. Surprisingly, it is
used in the food industry as it is a precursor to the compounds that are responsible for apple and
pineapple flavorings. Elemental analysis reveals a percent composition of 58.80% C, 9.87 % H and
31.33% O. What is the empirical formula for valeric acid? [15 pts]
58.80 g C x
1 mol
= 4.90 mol
= 2.5 x 2 = 5
1.96 mol
12.011 g
1 mol
= 9.79 mol
= 5 x 2 = 10
1.96 mol
1.0079 g
1 mol
31.33 g O x
= 1.96 mol
=1x2=2
1.96 mol
15.9996 g
The empirical formula is: C5 H10 O 2 (which also happens to be the molecular formula)
9.87 g H x
C5H10O2
4. A reaction requires lithium ions, and 350.0 mg Li3PO4 (molar mass = 115.79 g/mol) are added to the
reaction. How many individual ions of lithium are present for the reaction? [10 pts]
350.0 mg Li3 PO 4 x
1 mol Li3 PO 4
1g
3 mol Li
6.022 x 1023 ions
x
x
x
= 5.461 x 1021 ions
1000 mg
115.79 g
1 mol Li3 PO 4
1 mol
5.461 x 1021 ions
5. Ethyl acetate, C4H8O2, is the primary component of nail polish remover, and has a density of
0.897 g/mL. How many moles of ethyl acetate are contained within 750.0 mL of ethyl acetate?
[9 pts]
750.0 mL x
0.897 g
1 mol
x
= 7.64 mol
1 mL
88.11 g
7.64 mol ethyl acetate
Exam 3
Fall 2007
Page 2 of 4
Chemistry 100
Clark College
6. Use the solubility rules to determine if the following compounds are soluble (S) or insoluble (I). If
the compound is soluble, provide the number and type of ion(s) that will form. [20 pts]
a. BaCl2
If soluble, ions?
Ba+2 + 2 Cl-
Soluble
If soluble, ions?
NH4+ + MnO4-
Insoluble
If soluble, ions?
Soluble
If soluble, ions?
Insoluble
If soluble, ions?
Soluble?
Soluble
b. NH4MnO4
Soluble?
c. Cd(OH)2
Soluble?
d. CaI2
Soluble?
Ca+2 + 2 I-
e. Al2S3
Soluble?
Multiple Choice. Circle the choice that best answers the question/statement. [4 pts each]
7. Solid sodium phosphate is added to water. What statement best describes the resulting solution?
a. The solution is homogeneous, and the sodium phosphate remains bonded together.
b. The solution is heterogeneous, and the sodium phosphate remains bonded together.
c. The solution is homogeneous, and one Na3+3 ion and one PO4-3 ion “float” in solution.
d. The solution is homogeneous, and three Na+ ions and one PO4-3 ion “float” in solution.
8. What is the mass percent of carbon in butyric acid, C4H8O2, which is the compound responsible for
the rancid butter smell?
a. 13.63%
b. 27.27%
c. 54.53%
4 x 12.011 amu
x 100 = 54.53%
88.10 amu
d. 68.34%
Exam 3
Fall 2007
Page 3 of 4
Chemistry 100
Clark College
9. All monosaccharides (simple sugars) have the empirical formula of CH2O. Xylose is an example of
a monosaccharide, and is a wood sugar that is found in most edible plants. Xylose has a molar mass
of 150.13 g/mol, what is the molecular formula?
a. C4H8O4
FW = 30.03 g/mol
b. C5H10O5
n=
c. C6H12O6
d. C7H14O7
150.13 g/mol
=5
30.03 g/mol
(
Formula = CH 2 O
)
5
= C5 H10 O5
10. The number of moles in a 20.67 g sample of nitrogen gas is: Nitrogen gas is N2!
a. 0.7377 mol
b. 1.475 mol
20.67 g N 2 x
1 mol N 2
28.02 g
= 0.7377 mol N 2
c. 289.5 mol
d. 579.0 mol
Exam 3
Fall 2007
Page 4 of 4