Download Chemistry Activity: Determining the “weighted average” atomic mass

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Chemistry Activity: Determining the “weighted average” atomic mass of pennium
Pre-lab: Prior to 1982, pennies were made from a solid copper alloy. In 1982, the government
found that it was cheaper to make pennies from a zinc core and cover it with a thin layer
of the more expensive copper. Zinc is less dense than copper, so pre 1982 pennies
have more mass than the newer pennies. We will pretend that the element pennium is
made from penny atoms. The pre-1982 and post-1982 pennies represent the heavier and
lighter isotopes of the element pennium. Your job is to determine the “atomic mass” of pennium.
Procedure:
1. From the container of pennies at the front bench, reach in and grab a handful (about 30-50) pennies. Some will be
pre 1982, (the heavier isotope) and the others will be post 1982 (the lighter isotope of pennium).
2. On the back, make a data table for your measurements. Count and record the total number of pennies. Count and
record the number of each type of penny. Determine the percent of each type of penny (isotope). (3 pts total)
3. Find the mass of each isotope (type of penny). Do this by measuring all of the pennies of one type and dividing by the
total number of them. Find the average mass of each penny of each type. (2 pts total)
4. Please return pennies to the container.
Conclusion Questions:
1. What is the atomic mass of pennium? Show how you calculated this value in units of grams.
(2 pts)
2. How would the atomic mass of pennium be different if the % of pre-1982 pennies in your sample had been greater?
Explain. (1 pt)
3. Compare your results with at least 2 other lab groups. Explain why your results were not identically the same as the
other groups. (2 pts)
4. In the real world, two naturally-occurring samples of an element may not give exactly the same %’s of the isotopes
(although they’re usually fairly close.) Think of 2 reasons why this may be so. (2 pts)
5. Solve the following problems. Include these problems as part of your lab report.
a. Isotopes differ from each other in terms of ( nuclear charge / number of protons / chemical reactivity /
number of neutrons ). (1 pt)
b. What isotope of what element is the atomic mass standard? Write its symbol in the space below.
c.
How many neutrons are in one atom of
184
74W
?
(1 pt)
(1 pt)
d. The atomic mass for Zn is given as 65.39 amu (or grams / mole). Explain why there is no actual zinc atom with
an atomic mass of 65.39. (1 pt)
50
52
e. 4 isotopes of chromium (Cr) occur naturally. Their symbols and %’s are 24Cr (4.31%), 24Cr (83.76%),
54
(9.55%), and 24Cr 2.38%. Calculate the atomic mass of Cr from this data. Show your calculation. (1 pt)
53
24Cr
f.
Fill in the blank spaces in the following chart:
Symbol
Atomic
Number
Mass
Number
13
27
(4 pts total)
Number of
Protons
Number of
Neutrons
Number of
Electrons
10
137
56
56
17
18
17
32
-2
16S
16
17
18
g. Oxygen occurs in 3 naturally occurring isotopes: O, O, and O. The %’s are respectively 99.762%, 0.038%
and 0.200%. Calculate the average atomic mass of oxygen. (1 pt)
63
65
Bonus #1: The atomic mass of copper is 63.54 amu. It’s composed of 2 isotopes: 29Cu and 29Cu. Calculate the
relative abundances (the %’s) of each of these two isotopes in naturally-occurring samples. Show your
calculation. (1 pt)
Bonus #2: Naturally occurring boron (B) is 80.20% of 11B and 19.80% of some other isotope. If the average atomic mass of B is
10.81 amu, what is the mass number of the other isotope? Show your calculation. (1 pt)