Unit 2: C.9

1. Unit 2: C.9 In which you will learn about: Percent composition

2. 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

3. 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.

4. 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

5. 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%.

6. 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

7. 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.

8. 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.

9. Copper-Containing Minerals Chalcocite Chalcopyrite Malachite

10. 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

11. 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?

12. 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?

13. % 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.

14. 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%)

15. 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