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Average Atomic Mass

Solve the following problems, showing all work. If you show you work you can look back over it to study. Also, you can check to see whether you did it correctly. The answer is not as important as the work! Some problems require answers in words. Exercise your brain by putting the answers to these questions in the form of complete sentences.

- Carl’s grades for the first semester are Q
_{1}: 82, Q_{2}: 87, R_{1}: 84. What is his semester grade? - Millie would like to earn a 93 for the semester. If her Q
_{1}grade is 92 and her Q_{2}grade is 87 what grade does she need on the semester exam? - Eric’s first quarter grade was 84. He wants to know what grades he needs for Q
_{2}and R_{1}to earn a 90 for the semester. Assume that he will earn the same grade for Q_{2}and R_{1}. - The average atomic mass of magnesium (Mg) is 24.305 amu. The exact atomic mass of the magnesium-26 (
^{26}_{12}Mg) isotope is 25.982593 amu. Explain why there is a difference. - What is the meaning of the mass of an element as given in the periodic table? What does it have to do with the exact masses of the isotopes of an element?

Show your work for all the following calculations. Weights are atomic masses expressed in atomic mass units (amu). Write the symbol for each isotope where indicated.

- Find the average atomic mass of
potassium (K ).

Symbol Mass number Exact mass Percent abundance ^{39}_{19}K39 38.963707 93.2581 40 39.963998 0.0117 41 40.961826 6.7302 - Find the average atomic mass of sulfur
(S).

Symbol Mass number Exact weight Percent abundance 32 31.972070 95.02 33 32.971458 0.75 34 33.967866 4.21 36 35.967080 0.02

- Find the average atomic mass of tin (Sn).

Symbol Mass number Exact weight Percent abundance 112 111.904826 0.97 114 113.902784 0.65 115 114.903348 0.36 116 115.901747 14.53 117 116.902956 7.68 118 117.901609 24.22 119 118.903310 8.58 120 119.902200 32.59 122 121.903440 4.63 124 123.905274 5.79 - The element gallium (Ga) has two naturally occurring isotopes,
^{69}Ga and^{71}Ga, with an average atomic mass of 69.723 amu. Gallium is 60.109%^{69}Ga, and the atomic mass of^{71}Ga is 70.924707 amu. Calculate the mass of^{69}Ga. - The isotopes of argon are
^{36}_{18}Ar,^{38}_{18}Ar, and^{40}_{18}Ar. Argon-40 is by far the most abundant isotope of the three. Find the percent abundance of this isotope using the following information:Symbol Mass number Exact mass Percent abundance ^{36}_{18}Ar36 35.967546 0.336530 ^{38}_{18}Ar38 37.962732 0.06325 ^{40}_{18}Ar40 39.962383 - An element “X” has three major isotopes, which are listed below along with their abundances. Calculate the average atomic mass. What is the element?
Symbol Exact mass Percent abundance ^{20}X19.992440 90.48 ^{21}X20.993847 0.27 ^{22}X21.991386 9.25