Chemical Calculations And Chemical Formulas Worksheets With Answer Key Page 18
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Study Guide for An Introduction to Chemistry
Challenge Problems
95. Calamine is a naturally occurring zinc silicate that contains the equivalent of 67.5% zinc
oxide, ZnO. (The term calamine also refers to a substance used to make calamine lotion.)
What is the mass, in kilograms, of zinc in 1.347 10
4
kg of natural calamine that is 67.5%
ZnO?
3
67.5 kg ZnO
10 g
1 mol ZnO
1 mol Zn
4
? kg Zn = 1.347 × 10 kg calamine
100 kg calamine
1 kg
81.39 g ZnO
1 mol ZnO
65.39 g Zn
1 kg
3
=
7.30 10 kg Zn
3
1 mol Zn
10 g
97. Flue dust from the smelting of copper and lead contains As
O
. (Smelting is the heating of a
2
3
metal ore until it melts, so that its metallic components can be separated.) When this flue dust
is collected, it contains 90% to 95% As
O
. The arsenic in As
O
can be reduced to the
2
3
2
3
element arsenic by reaction with charcoal. What is the maximum mass, in kilograms, of
arsenic that can be formed from 67.3 kg of flue dust that is 93% As
O
?
2
3
3
93 kg As O
10 g
1 mol As O
2 mol As
2
3
2
3
? kg As = 67.3 kg flue dust
100 kg flue dust
1 kg
197.841 g As O
1 mol As O
2
3
2
3
74.9216 g As
1 kg
3
1 mol As
10 g
93 kg As O
2 74.9216 g As
2
3
or
? kg As = 67.3 kg flue dust
= 47 kg As
100 kg flue dust
197.841 g As O
2
3
99. Magnesium metal, which is used to make die-cast auto parts, missiles, and space vehicles, is
obtained by the electrolysis of magnesium chloride. Magnesium hydroxide forms magnesium
chloride when it reacts with hydrochloric acid. There are two common sources of magnesium
hydroxide.
a. Magnesium ions can be precipitated from seawater as magnesium hydroxide,
Mg(OH)
. Each kiloliter of seawater yields about 3.0 kg of the compound. How many
2
metric tons of magnesium metal can be made from the magnesium hydroxide derived
from 1.0 10
5
kL of seawater?
3
3.0 kg Mg(OH)
10 g
1 mol Mg(OH)
5
2
2
? t Mg = 1.0 × 10 kL seawater
1 kL seawater
1 kg
58.3197 g Mg(OH)
2
1 mol Mg
24.3050 g Mg
1 t
2
=
1.3 10 t Mg
6
1 mol Mg(OH)
1 mol Mg
10 g
2
b. Brucite is a natural form of magnesium hydroxide. A typical crude ore containing
brucite is 29% Mg(OH)
. What minimum mass, in metric tons, of this crude ore is
2
necessary to make 34.78 metric tons of magnesium metal?
6
10 g
1 mol Mg
1 mol Mg(OH)
2
? t ore = 34.78 t Mg
1 t
24.3050 g Mg
1 mol Mg
58.3197 g Mg(OH)
1 t
100 t ore
2
2
=
2.9 10 t ore
6
1 mol Mg(OH)
10 g
29 t Mg(OH)
2
2
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