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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.

  1. Carl’s grades for the first semester are Q1: 82, Q2: 87, R1: 84. What is his semester grade?
  2. Millie would like to earn a 93 for the semester. If her Q1 grade is 92 and her Q2 grade is 87 what grade does she need on the semester exam?
  3. Eric’s first quarter grade was 84. He wants to know what grades he needs for Q2 and R1 to earn a 90 for the semester. Assume that he will earn the same grade for Q2 and R1.
  4. The average atomic mass of magnesium (Mg) is 24.305 amu. The exact atomic mass of the magnesium-26 (2612Mg) isotope is 25.982593 amu. Explain why there is a difference.
  5. 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.

  1. Find the average atomic mass of potassium (K ).
    Symbol Mass number Exact mass Percent abundance
    3919K 39 38.963707 93.2581
      40 39.963998 0.0117
      41 40.961826 6.7302
  2. 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

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  1. 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
  2. The element gallium (Ga) has two naturally occurring isotopes, 69Ga and 71Ga, with an average atomic mass of 69.723 amu. Gallium is 60.109% 69Ga, and the atomic mass of 71Ga is 70.924707 amu. Calculate the mass of 69Ga.
  3. The isotopes of argon are3618Ar,3818Ar, and4018Ar. 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
    3618Ar 36 35.967546 0.336530
    3818Ar 38 37.962732 0.06325
    4018Ar 40 39.962383  
  4. 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
    20X 19.992440 90.48
    21X 20.993847 0.27
    22X 21.991386 9.25
This homework belongs with the Group Activity of the
same name: Average Atomic Mass. Refer to it for help completing this page.
Data for this worksheet came from: The Pictorial Period Table and from ChemTeam
For this packet to be complete, print out one copy of the periodic table for each student.
Last updated: Oct 30, 2012       Home