Chapter 3: Stoichiometry
1. The atomic mass of rhenium is 186.2. Given that 37.1% of natural rhenium is rhenium-185,
what is the other stable isotope?
A)
B)
C)
D)
E)
2. Consider the element indium, atomic number 49, atomic mass 114.8 g. The nucleus of an
atom of indium-112 contains
A)
49 protons, 63 neutrons, 49 electrons
B)
49 protons, 49 neutrons
C)
49 protons, 49 alpha particles
D)
49 protons, 63 neutrons
E)
49 protons, 112 neutrons
3. A hypothetical element consists of two isotopes of masses 69.95 amu and 71.95 amu with
abundances of 25.7% and 74.3%, respectively. What is the average atomic mass of this
element?
A)
70.95 amu
B)
69.95 amu
C)
70.5 amu
D)
71.4 amu
E)
71.95 amu
4. Naturally occurring copper exists in two isotopic forms: 63Cu and 65Cu. The atomic mass of
copper is 63.55 amu. What is the approximate natural abundance of 63Cu?
A)
63%
B)
90%
C)
70%
D)
50%
E)
30%
5. Naturally occurring element X exists in three isotopic forms: X-28 (27.977 amu, 92.23%
abundance), X-29 (28.976 amu, 4.67% abundance), and X-30 (29.974 amu, 3.10%
abundance). Calculate the atomic weight of X.
A)
28.09 amu
B)
48.63 amu
C)
27.16 amu
D)
28.97 amu
E)
86.93 amu
6. Naturally occurring element X exists in three isotopic forms: X-28 (27.977 amu, 92.23%
abundance), X-29 (28.976 amu, 4.67% abundance), and X-30 (29.974 amu, 3.10%
abundance). What is the identity of element X?
A)
Cu
B)
Al
C)
Ni
D)
Si
E)
Sr
7. The average mass of a carbon atom is 12.011. Assuming you were able to pick up only one
carbon unit, the chances that you would randomly get one with a mass of 12.011 is
A)
0%
B)
0.011%
C)
about 12%
D)
12.011%
E)
greater than 50%
8. What is the mass of 4 atom(s) of copper in grams?
A)
254.2 g
B)
2.37 1021 g
C)
9.57 10–24 g
D)
6.022 1023 g
E)
4.22 10–22 g
9. Iron is biologically important in the transport of oxygen by red blood cells from the lungs to
the various organs of the body. In the blood of an adult human, there are approximately
2.64 1013 red blood cells with a total of 2.90 g of iron. On the average, how many iron
atoms are present in each red blood cell? (molar mass Fe = 55.85 g/mol)
A)
8.44 10–10
B)
1.18 109
C)
3.13 1022
D)
2.64 1013
E)
6.14 10–2
10. You have a sample of zinc (Zn) and a sample of aluminum (Al). You have an equal number
of atoms in each sample. Which of the following statements concerning the masses of the
samples is true?
A)
The mass of the zinc sample is more than twice as great as the mass of the
aluminum sample.
B)
The mass of the zinc sample is more than the mass of the aluminum sample, but it
is not twice as great.
C)
The mass of the aluminum sample is more than twice as great as the mass of the
zinc sample.
D)
The mass of the aluminum sample is more than the mass of the zinc sample, but it
is not twice as great.
E)
The masses of each sample are equal.
11. A sample of ammonia has a mass of 43.5 g. How many molecules are in this sample?
A)
2.55 molecules
B)
2.62 1025 molecules
C)
2.36 1023 molecules
D)
1.54 1024 molecules
E)
8.63 10–16 molecules
12. How many moles of hydrogen sulfide are contained in a 49.7-g sample of this gas?
A)
0.686 mol
B)
1.46 mol
C)
83.8 mol
D)
24.7 mol
E)
2.92 mol
13. What is the molar mass of ethanol (C2H5OH)?
A)
45.06 g/mol
B)
34.06 g/mol
C)
46.07 g/mol
D)
30.03 g/mol
E)
105.03 g/mol
14. For which compound does 0.256 mole weigh 12.8 g?
A)
C2H4O
B)
CO2
C)
CH3Cl
D)
C2H6
E)
none of these
15. Roundup, an herbicide manufactured by Monsanto, has the formula C3H8NO5P. How many
moles of molecules are there in a 295.1-g sample of Roundup?
A)
0.5729
B)
2.137
C)
1.745
D)
16.39
E)
none of these
16. Calculate the molar mass of barium sulfite.
A)
233.40 g/mol
B)
201.40 g/mol
C)
354.73 g/mol
D)
514.87 g/mol
E)
217.40 g/mol
17. Phosphorus has the molecular formula P4, and sulfur has the molecular formula S8. How
many grams of phosphorus contain the same number of molecules as 4.23 g of sulfur?
A)
2.04 g
B)
0.490 g
C)
4.08 g
D)
4.23 g
E)
none of these
18. A given sample of a xenon fluoride compound contains molecules of a single type XeFn,
where n is some whole number. Given that 8.06 1020 molecules of XeFn weigh 0.227 g,
calculate n.
A)
1
B)
6
C)
4
D)
3
E)
2
19. Phosphoric acid can be prepared by reaction of sulfuric acid with “phosphate rock”
according to the equation:
Ca3(PO4)2 + 3H2SO4 → 3CaSO4 + 2H3PO4
What is the molar mass of Ca3(PO4)2?
A)
310.18 g/mol
B)
87.05 g/mol
C)
278.18 g/mol
D)
215.21 g/mol
E)
166.02 g/mol
20. Phosphoric acid can be prepared by reaction of sulfuric acid with “phosphate rock”
according to the equation:
Ca3(PO4)2 + 3H2SO4 → 3CaSO4 + 2H3PO4
How many oxygen atoms are there in 1.75 ng of Ca3(PO4)2?
A)
3.40 1012
B)
1.36 1013
C)
8.43 1015
D)
2.72 1022
E)
2.72 1013
21. What is the mass of a 6.761-mol sample of sodium hydroxide?
A)
40.00 g
B)
270.4 g
C)
162.3 g
D)
5.916 g
E)
0.1690 g
22. How many atoms of hydrogen are present in 7.63 g of ammonia?
A)
2.70 1023
B)
1.52 1024
C)
1.38 1025
D)
8.09 1023
E)
1.12 1020
23. The molar mass of the compound formed by potassium and selenium is
A)
157.2 g/mol
B)
197.0 g/mol
C)
118.1 g/mol
D)
276.0 g/mol
E)
196.3 g/mol
24. What is the molar mass of cryolite (Na3AlF6)?
A)
209.9 g/mol
B)
185.3 g/mol
C)
210.0 g/mol
D)
104.2 g/mol
E)
68.97 g/mol
25. One molecule of a compound weighs 2.93 10–22 g. The molar mass of this compound is:
A)
2.06 g/mol
B)
567 g/mol
C)
168 g/mol
D)
176 g/mol
E)
none of these
26. NaHCO3 is the active ingredient in baking soda. How many grams of oxygen are in 0.52 g
of NaHCO3?
A)
0.099 g
B)
0.019 g
C)
6.19 103 g
D)
0.033 g
E)
0.30 g
27. A compound is composed of element X and hydrogen. Analysis shows the compound to be
80% X by mass, with three times as many hydrogen atoms as X atoms per molecule. Which
element is element X?
A)
He
B)
C
C)
F
D)
S
E)
none of these
28. Which compound contains the highest percent by mass of hydrogen?
A)
HCl
B)
H2O
C)
H2SO4
D)
H2S
E)
HF
29. A substance contains 35.0 g nitrogen, 5.05 g hydrogen, and 60.0 g of oxygen. How many
grams of hydrogen are there in a 153-g sample of this substance?
A)
7.72 g
B)
767 g
C)
15.4 g
D)
5.05 g
E)
30.3 g
30. How many grams of potassium are in 27.8 g of K2CrO7?
A)
4.49 g
B)
1.422 g
C)
8.98 g
D)
78.2 g
E)
55.6 g
31. Chlorous acid, HClO2, contains what percent hydrogen by mass?
A)
1.92%
B)
25.0%
C)
23.4%
D)
1.47%
E)
5.18%
32. A mixture of KCl and KNO3 is 44.20% potassium by mass. The percentage of KCl in the
mixture is closest to
A)
40%
B)
50%
C)
60%
D)
70%
E)
80%
33. A substance, A2B, has the composition by mass of 60% A and 40% B. What is the
composition of AB2 by mass?
A)
40% A, 60% B
B)
50% A, 50% B
C)
27% A, 73% B
D)
33% A, 67% B
E)
none of these
34. Each molecule of testosterone contains 19 atoms of carbon (plus other atoms). The mass
percent of carbon in testosterone is 79.12%. What is the molar mass of testosterone?
A)
576.8 g/mol
B)
180.5 g/mol
C)
228.2 g/mol
D)
240.1 g/mol
E)
288.4 g/mol
35. A substance contains 23.0 g sodium, 27.0 g aluminum, and 114 g fluorine. How many
grams of sodium are there in a 102-g sample of the substance?
A)
4.43 g
B)
23.0 g
C)
14.3 g
D)
102 g
E)
none of these
36. An oxide of iron has the formula Fe3O4. What mass percent of iron does it contain?
A)
0.72%
B)
28%
C)
30%
D)
70%
E)
72%
37. The mineral vanadinite has the formula Pb5(VO4)3Cl. What mass percent of chlorine does it
contain?
A)
7.51 %
B)
3.08 %
C)
9.91 %
D)
2.50 %
E)
1.13 %
38. Which of the following compounds has the same percent composition by mass as styrene,
C8H8?
A)
acetylene, C2H2
B)
benzene, C6H6
C)
cyclobutadiene, C4H4
D)
-ethyl naphthalene, C12H12
E)
all of these
39. The molar mass of an insecticide, dibromoethane, is 187.9 g/mol. Its molecular formula is
C2H4Br2. What percent by mass of bromine does dibromoethane contain?
A)
42.52%
B)
2.14%
C)
85.05%
D)
12.78%
E)
6.39%
40. Hydrocortisone valerate is an ingredient in hydrocortisone cream, prescribed for skin
problems. Its molecular formula is C26H38O6. What is the percent by mass of carbon in
hydrocortisone valerate?
A)
69.9%
B)
43.6%
C)
54.2%
D)
76.9%
E)
60.1%
41. Ammonium chromate, (NH4)2CrO4, contains what percent nitrogen by mass?
A)
36.8%
B)
9.2%
C)
18.4%
D)
11.9%
E)
none of these
42. You heat 3.854 g of a mixture of Fe3O4 and FeO to form 4.148 g Fe2O3. The mass percent of
FeO originally in the mixture was:
A)
92.9%
B)
55.8%
C)
44.2%
D)
38.5%
E)
none of these
43. You take an aspirin tablet (a compound consisting solely of carbon, hydrogen, and oxygen)
with a mass of 1.00 g, burn it in air, and collect 2.20 g of carbon dioxide and 0.400 g water.
The molar mass of aspirin is between 170 and 190 g/mol. The molecular formula of aspirin
is
A)
C6H8O5
B)
C9H8O4
C)
C8H10O5
D)
C10H6O4
E)
none of these
44. Suppose you are given the percent by mass of the elements in a compound and you wish to
determine the empirical formula. Which of the following is true?
A)
You must convert percent by mass to relative numbers of atoms.
B)
You must assume exactly 100.0 g of the compound.
C)
You must divide all of the percent by mass numbers by the smallest percent by
mass.
D)
You cannot solve for the empirical formula without the molar mass.
E)
At least two of the above (A-D) are true.
45. A chloride of rhenium contains 63.6% rhenium. What is the formula of this compound?
A)
ReCl
B)
ReCl3
C)
ReCl5
D)
ReCl7
E)
Re2Cl3
46. A 2.80-g sample of an oxide of bromine is converted to 4.698 g of AgBr. Calculate the
empirical formula of the oxide. (molar mass for AgBr = 187.78 g/mol)
A)
BrO3
B)
BrO2
C)
BrO
D)
Br2O
E)
none of these
47. A hydrocarbon (a compound consisting solely of carbon and hydrogen) is found to be
85.6% carbon by mass. What is the empirical formula for this compound?
A)
CH
B)
CH2
C)
C2H
D)
C3H
E)
CH4
48. The empirical formula of a group of compounds is CHCl. Lindane, a powerful insecticide, is
a member of this group. The molar mass of lindane is 290.8 g/mol. How many atoms of
carbon does a molecule of lindane contain?
A)
2
B)
3
C)
4
D)
6
E)
8
49. The empirical formula of styrene is CH; its molar mass is 104.1 g/mol. What is the
molecular formula of styrene?
A)
C2H4
B)
C8H8
C)
C10H12
D)
C6H6
E)
none of these
50. Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical
formula?
A)
C3H5O2
B)
C3H3O4
C)
C2HO3
D)
C2H5O4
E)
C3HO3
51. Vitamin C contains the elements C, H, and O. It is known to contain 40.9% C and 4.58% H
by mass. The molar mass of vitamin C has been found to be about 180 g/mol. The molecular
formula for vitamin C is:
A)
C2H3O2
B)
C3H4O3
C)
C4H6O4
D)
C6H8O6
E)
C14H18
52. A compound composed of sulfur and fluorine is found to contain 62.79% by mass of sulfur.
If the molar mass of the compound is 102.13 g/mol, what is its molecular formula?
A)
S3F
B)
S2F4
C)
SF
D)
S2F
E)
S2F2
53. A 0.4987-g sample of a compound known to contain only carbon, hydrogen, and oxygen
was burned in oxygen to yield 0.9267 g of CO2 and 0.1897 g of H2O. What is the empirical
formula of the compound?
A)
CHO
B)
C2H2O
C)
C3H3O2
D)
C6H3O2
E)
C3H6O2
54. Oxides of copper include CuO and Cu2O. You heat 1.51 g of one of these copper oxides in
the absence of air and obtain 1.21 g of Cu.
True or false: You must have had CuO.
55. The molecular formula always represents the total number of atoms of each element present
in a compound.
56. How many of the following are true concerning balanced chemical equations?
I.
The number of molecules is conserved.
II.
The coefficients for the reactants tell you how much of each reactant you are given.
III.
Atoms are neither created nor destroyed.
IV.
The coefficients indicate the mass ratios of the substances used.
V.
The sum of the coefficients on the reactant side equals the sum of the coefficients on
the product side.
A)
1
B)
2
C)
3
D)
4
E)
5
57. Balanced chemical equations imply which of the following?
A)
Numbers of molecules are conserved in chemical change.
B)
Numbers of atoms are conserved in chemical change.
C)
Volume is conserved in chemical change.
D)
A and B
E)
B and C
58. In balancing an equation, we change the __________ to make the number of atoms on each
side of the equation balance.
A)
formulas of compounds in the reactants
B)
coefficients of compounds
C)
formulas of compounds in the products
D)
subscripts of compounds
E)
none of these
59. What is the coefficient for water when the following equation is balanced?
As(OH)3(s) + H2SO4(aq) → As2(SO4)3(aq) + H2O(l)
A)
1
B)
2
C)
4
D)
6
E)
12
60. Which of the following equations is not balanced?
A)
4Al + 3O2 → 2Al2O3
B)
C2H6 + O2 → 2CO2 + 3H2O
C)
2KClO3 → 2KCl + O2
D)
4P4 + 5S8 → 4P4S10
E)
P4 + 5O2 → P4O10
Consider a specific chemical reaction represented by the equation aA + bB → cC + dD. In
this equation the letters A, B, C, and D represent chemicals, and the letters a, b, c, and d
represent coefficients in the balanced equation.
61. How many possible values are there for the quantity “c/d”?
A)
1
B)
2
C)
3
D)
4
E)
infinite
62. How many possible values are there for the quantity “c”?
A)
1
B)
2
C)
3
D)
4
E)
infinite
63. What is the coefficient for oxygen when the following equation is balanced?
NH3(g) + O2(g) → NO2(g) + H2O(g)
A)
3
B)
6
C)
7
D)
12
E)
14
64. How many of the following statements are true concerning chemical equations?
I. Coefficients can be fractions.
II. Subscripts can be fractions.
III. Coefficients represent the relative masses of the reactants and/or products.
IV. Changing the subscripts to balance an equation can only be done once.
V. Atoms are conserved when balancing chemical equations.
A)
1
B)
2
C)
3
D)
4
E)
5
65. Determine the coefficient for O2 when the following equation is balanced in standard form
(smallest whole numbers).
C8H18(g) + O2(g) → CO2(g) + H2O(g)
A)
8
B)
17
C)
18
D)
25
E)
16
66. What is the sum of the coefficients of the following equation when it is balanced using
smallest whole numbers?
NaNH2 + NaNO3 → NaN3 +NaOH + NH3
A)
5
B)
6
C)
7
D)
8
E)
9
67. wPCl5 + xH2O → yPOCl3 + zHCl
The above equation is properly balanced when:
A)
w = 1, x = 2, y = 2, z = 4
B)
w = 2, x = 2, y = 2, z = 2
C)
w = 2, x = 2, y = 2, z = 1
D)
w = 1, x = 1, y = 1, z = 2
E)
none of these
68. Potassium forms an oxide containing 1 oxygen atom for every 2 atoms of potassium. What
is the coefficient of oxygen in the balanced equation for the reaction of potassium with
oxygen to form this oxide?
A)
0
B)
1
C)
2
D)
3
E)
4
69. Indium reacts with chlorine to form InCl3. In the balanced equation for this reaction, the
coefficient of the indium trichloride is
A)
1
B)
2
C)
3
D)
4
E)
6
70. Give (in order) the correct coefficients to balance the following reaction:
H2SnCl6 + H2S → SnS2 + HCl
A)
1, 2, 1, 6
B)
1, 2, 2, 2
C)
1, 1, 1, 6
D)
6, 2, 1, 1
E)
2, 4, 2, 6
71. In the balanced equation for the reaction:
xP4O6(s) + yH2O(l) → zH3PO3(aq)
if x equals 2, the coefficient z equals:
A)
2
B)
4
C)
6
D)
10
E)
none of these
72. The correct coefficients to balance the following reaction are, in order:
I2O5 + H2S → I2 + SO2 + H2O
A)
1, 1, 1, 1, 1
B)
1, 2, 1, 2, 1
C)
2, 4, 1, 4, 5
D)
2, 5, 2, 5, 5
E)
3, 5, 3, 5, 5
73. When the equation C6H14 + O2 → CO2 + H2O is balanced with the smallest set of integers,
the sum of the coefficients is
A)
4
B)
47
C)
15
D)
27
E)
34
74. Which of the following equations correctly describes the combustion of CH4 and O2 to
produce water (H2O) and carbon dioxide (CO2)?
A)
CH4 + O2 → CO2 + H2O
B)
CH4 + O2 → CO2 + 2H2O
C)
CH4 + 2O2 → CO2 + 2H2O
D)
CH4 + 3O2 → 2CO2 + H2O
E)
2CH4 + 3O2 → 2CO2 + 2H2O
75. A reaction occurs between sodium carbonate and hydrochloric acid producing sodium
chloride, carbon dioxide, and water. The correct set of coefficients, respectively, for the
balanced reaction is: