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 MineralsSOME 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