Pennies minted after 1982 were made of zinc with a thin layer of copper plated on top. The zinc can be dissolved out leaving a copper shell. We conducted an experiment to explore this and then were given a test. Here is the test, with answers and the logic to understand HOW to answer the questions:
1. Pennies minted from 1962 to 1982 were an alloy of 95% copper and 5% zinc.
a. (5) If one of the pennies weighs 3.1 grams, how many grams of copper is in it?
b. (5) How many grams of zinc is in the penny?
Ans: An "alloy" is a usually considered a mixture of two or more metals. Steel is a mixture of iron and carbon. Brass is a mixture of copper and zinc. Bronze is a mixture of copper and tin.
You are told that copper is 95% of the coin and zinc is the remaining 5%. The percentages must add up to 100%. This means that 95% of 3.1 grams is the amount of copper in the coin, and 5% of 3.1 grams is the amount of zinc in the coin. This means that the whole coin -- 3.1 grams -- represents 100% of the mass of it. To find the amount of copper, set up a proportion like this: 100%/95% = 3.1 grams/x. Cross multiply to obtain
(100%)(x) = (3.1 grams)(95%).
Then divide both sides by 100% to get "x" by itself:
x = (3.1 grams)(95%)/100%
Performing the arithmetic you find x = 2.945 grams of copper. Remember, the "%" signs cancel out and the units that are left are grams. Since 95% is a part of 100% -- it is LESS than 100% -- the number of grams you get have to be less than 3.1. If you get MORE than 3.1, you have done something wrong.
To find the amount of zinc in the penny, subtract 2.945 grams from 3.1 grams:
3.1 grams - 2.945 grams = 0.155 grams. Since there are two digits in the least accurate factor (3.1), we round off the answer to 016 grams of zinc.
2. Pennies minted from 1983 to the present consist of a core of zinc and a copper cladding. The penny weighs 2.4 grams. Zinc is supposed to be 97.5% of the weight of the penny. Copper is supposed to be the balance.
a. (5) What percent of copper is present in these pennies?
b. (5) What is the difference between an "alloy" penny and a "clad" penny?
Ans: You are told that zinc is 97.5% of the mass of the coin. To find the percent of copper in this penny, subtract 97.5% from 100% and get 2.5%. Notice that the question does NOT ask for the number of grams of copper; just the PERCENT.
In class you were told that an alloy is a mixture of metals. "Clad" means a coating or covering over something. For example, "When the house caught on fire, Jenny ran out CLAD only in her pajamas."
3. When zinc is placed in an acid, it reacts to form a zinc salt and a gas. The formula for the reaction of zinc with hydrogen chloride (which is found in hydrochloric acid) is:
Zn + 2HCl --> ZnCl2 + H2
a. (5) What is the name of the salt formed?
b. (5) What is the name of the gas formed?
ANS: You were told in class that the most common salt is table salt, or sodium chloride. It is composed of a metal (sodium) and a non metal (chlorine). You were told that any metal and non metal are called "salts." This is the name of a class of compound. So the salt formed from zinc and chlorine is named "zinc chloride." There is no sodium in the compound. You can see this by looking at the chemical equation. You were also taught that hydrogen has the symbol "H" and that two atoms of hydrogen form a molecule of hydrogen gas, (H2).
4. After soaking the penny in question 2 in hydrochloric acid for two days, Raquel found that the zinc had all disappeared. She carefully washed and dried what remained of the penny; it was a thin, copper shell. When she weighed it, she discovered that it was 0.07 gram. (Raquel had to estimate the answer because the balance was only calibrated in tenths of grams.)
a. (5) What percent of the coin remained?
b. (5) What percent of zinc reacted?
c. (5) What percent of the coin was zinc?
d. (5) How do these percentages (of zinc and of copper) compare to the percentages stated (and calculated) in question 2? ("The 1983 - current penny is said to be ______% Zn and ______% Cu. Raquel's results indicated the penny was _____% Zn and _____% Cu. Based on these results Raquel's penny contained _________ (more/less) copper than was predicted from the data.")
ANS: (part a) You are told that 0.07 grams copper remains. To find the percent, set up a proportion:
100%/2.4 grams = x/0.07 grams.
Cross multipy: (100%)(0.07 grams) = (2.4 grams)(x)
Divide both sides by 2.4 grams to get the unknown "x" by itself.
(100%)(0.07 grams)/(2.4 grams) = x
The grams units cancel. Divide 0.07 by 2.4 and multiply by 100% to find 2.92% copper.
(part b) Since ALL of the zinc reacted, 100% of the zinc is gone.
(part c) Since 2.92% of the coin was copper, the zinc must be 100% - 2.92% or 97.08%. You could also set up another proportion: 100%/2.4 grams = x/(2.4 grams - 0.07 grams). Then (100%)(2.33 grams) = (2.4 grams)(x) and
(100%)(2.33 grams)/(2.4 grams) = x = 97.08%
(part d) All you had to do was to substitute the numbers you found in the spaces provided and compare the results: "The 1983 - current penny is said to be _97.5_____% Zn and __2.5____% Cu. Raquel's results indicated the penny was _97.08____% Zn and _2.92____% Cu. Based on these results Raquel's penny contained ___more______ (more/less) copper than was predicted from the data.")
5. After soaking in the HCl, some of the pennies looked silvery. Why?
ANS: Because some of the zinc that was inside the penny had to have formed zinc chloride. The zinc chloride was in solution. The amount of hydrochloric acid must have been very low, some of the water or HCl gas evaporated over the weekend, and the zinc precipitated out of solution. [All you needed to write was that the zinc from inside the penny somehow deposited on the outside.]
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