Unit 2: C.9
In which you will learn about:
•Percent composition
Composition of Materials
• In considering whether an ore is a useful
source of a metal, the percent composition of
the ore is a key factor
• Since separating a metal from its ore can be
costly (both energetically and financially), it is
desirable to identify ores with the highest
percent of metal for mining and processing
Coin Design
• One decision you will make when designing
your coin is whether to use only one material
or a combination of materials.
• If your design uses more than one material,
you will need to specify how much of each
material will be present in the coin.
• The percent by mass of each material found in
an item such as a coin is called its percent
composition.
Pennies
• You learned that the composition of the U.S.
penny has changed several times
• 1943 = zinc-coated steel
• 1943-1982 = mostly copper
• 1982+ = mostly zinc with a copper coating
Sample Problem
• A post-1982 penny with a mass of 2.500 g is
composed of 2.4375 g zinc and 0.0625 g
copper. What is its percent composition?
– The percent composition of the penny can be
found by dividing the mass of each constituent
material by the total mass of the penny and
multiplying by 100%.
Solution
• 2.4375 g zinc x 100% = 97.50% zinc
2.500 g total
• 0.062 g copper x 100% = 2.50% copper
2.500 g total
Geologists
• The idea of percent composition helps
geologists describe how much metal or
mineral is present in a particular ore.
• They can then evaluate whether the ore
should be mined and how it should be
processed.
Copper Minerals
• A compound’s formula indicates the relative number of
atoms of each element present in the substance.
• For example, one common commercial source of
copper metal is the mineral chalcocite, copper (I)
sulfide (Cu2S).
• Is formula indicates that the mineral contains twice as
many copper atoms as sulfur atoms.
• The formula also reveals how much copper can be
extracted from a certain mass of the mineral, which is
an important consideration in copper mining and
production.
Copper-Containing Minerals
SOME COPPER CONTAINING MINERALS
Common Name
Formula
Chalcocite
Cu2S
Chalcopyrite
CuFeS2
Malachite
Cu2CO3(OH)2
Chalcocite
Chalcopyrite
Malachite
Chalcocite % Composition Example
• The formula for chalcocite indicates that one
mole of Cu2S contains two moles of Cu, or 127
g Cu and one mole of S, or 32.1 g S.
• The molar mass of Cu2S is (2 x 63.5 g ) + 32.1 g
= 159 g/mol. Therefore,
• % Cu = total mass Cu/total mass Cu2S x 100%
• 127 g Cu x 100% = 79.9% Cu
159 g Cu2S
What about the %S?
• A second calculation indicates that Cu2S
contains 20.1 % sulfur.
• The sum of the percent copper and the
percent sulfur equals 100.0%.
• Why?
Other Considerations
• Knowing the percent composition of metal in
a particular mineral helps us decide whether a
particular ore should be mined; however, it is
not the sole criterion.
• What else do we need to consider?
% Mineral
• Suppose an ore contains the mineral chalcocite, Cu2S.
• Because nearly 80% of this mineral is Cu (see the
calculation), it seems likely that this ore is worth mining for
copper.
• However, we must also consider the quantity of mineral
actually contained in the ore.
• All other factors being equal, an ore that contains only 10%
chalcocite would be a less desirable copper source than
one containing 50% chalcocite.
• Thus, two factors must be taken into account when
deciding on the quality of a particular ore source: (1) the
percent mineral in the ore and (2) the percent metal in the
mineral.
A Visual Aid
This square represents the total
ore sample. Each colored square
represents one piece of
chalcocite (10%)
This square represents one piece
of chaclocite within the ore. The
colored squares here represent
the amount of Cu (80%)
HOMEWORK QUESTIONS
• 1) In carbon dioxide, two-thirds of the atoms
are oxygen atoms; however, the percent
oxygen by mass is not 67%. Explain.
• 2) Find the percent metal (by mass) in each of
the following compounds:
A. Ag2S
B. Al2O3
C. CaCO3