Clicker Questions: Wednesday, February 1
Please see my note on Question #5 below
1) How many electrons would it take to equal the same mass as the nuclei of two atoms of neon-22?
A) 40,317
C) 80,635
E) 40,323
B) 80,645
D) 79,328
F) 39,664
2) Based on your reading for today, which of the following statements is false?
A) The atomic mass for carbon on our periodic table is 12.01 g.
B) The atomic mass is an average mass for an element, accounting for the masses of its
isotopes and their abundances.
C) Any given element may have many different isotopes, but only 1 atomic number.
D) Some isotopes are not stable and can transform into other elements.
E) For an element having 2 different isotopes, its atomic mass will be closer to the mass of the
most commonly occurring isotope.
3) Based on what you learned last class, answer the following question: Over the weekend, Jeff
created a new element (Z = 130) and named it jeffrium (symbol = Jf). A sample of 10 atoms of
Jf indicating the percent abundance of its 2 isotopes is shown in the box below. What atomic
mass should be written on the periodic table for Jf?
KEY
 Jf-335 (334.9 amu)
 Jf-338 (337.9 amu)





A)
B)
130 amu
335.8 amu
C)
D)
E)
F)
336.4 amu
337.0 amu
337.4 amu
338.2 amu





4) Silver has two naturally occurring isotopes: Ag-107 and Ag-109. If Ag-107 has a mass of
106.905 amu and an abundance of 51.84%, what is the mass of Ag-109?
A) 107.87 amu
D) 55.42 amu
B) 108.97 amu
E) 52.49 amu
C) 52.45 amu
F) 108.84 amu
5)
We didn’t have time to do problem #5 in class.
Please work on it in preparation for next lecture and we’ll go over it next class.
Rubidium has two isotopes: Rb-85 (mass = 84.9118 amu) and Rb-87 (mass = 86.9092 amu).
What is the % abundance of the lighter isotope?
Answers: 1) B*
2) A
3) D*
* See answer worked out on next page
4) B*
5) answer provided after next class
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Clicker Questions: Wednesday, February 1
Selected answers with work shown
1) How many electrons would it take to equal the same mass as the nuclei of two atoms of neon-22?
Answer:
mass of two neon-22 nuclei = 2 x [10(mass proton) + 12(mass neutron)]
= 2 x [10(1.0073) + 12(1.0087)]
= 2 x [10.0730 + 12.1044]
= 2 x [22.1774]
= 44.3548 amu
Divide the mass of the two neon-22 nuclei by the mass of an electron:
= (44.3548 amu)/(0.00055 amu/e-)
= 80,645 electrons
3) Based on what you learned last class, answer the following question: Over the weekend, Jeff
created a new element (Z = 130) and named it jeffrium (symbol = Jf). A sample of 10 atoms of
Jf indicating the percent abundance of its 2 isotopes is shown in the box below. What atomic
mass should be written on the periodic table for Jf?
Answer:
3 of the 10 atoms in the drawing (i.e. 30%) are Jf-235 (334.9 amu); the other 7 of
the 10 atoms (i.e. 70%) are Jf-238 (337.9 amu)
atomic mass
= (0.30)(334.9 amu) + (0.70)(337.9 amu)
= 100.47 amu + 236.53 amu
= 337.0 amu
4) Silver has two naturally occurring isotopes: Ag-107 and Ag-109. If Ag-107 has a mass of
106.905 amu and an abundance of 51.84%, what is the mass of Ag-109?
Answer: If we just fill in what is in the question, it looks like there isn't enough information:
atomic mass = (51.84/100)(106.905 amu) + (fraction)(X)
However, we are expected to look at the periodic table to find the atomic mass and to realize that
the two % add up to 100%). That gives:
107.9 amu = (51.84/100)(106.905 amu) + (48.16/100) (X)
107.9 amu = 55.420 amu + 0.4816X
52.48 amu = 0.4816X
X = 108.97 amu
2