Calculating Average Atomic Mass Worksheet

By | November 15, 2025

Understanding atomic mass is fundamental to comprehending chemistry. However, the atomic mass listed on the periodic table isn’t simply the mass of a single atom of that element. Instead, it’s the *average atomic mass*, which takes into account the existence of different isotopes of the same element. These isotopes have the same number of protons but differ in the number of neutrons, leading to variations in their atomic mass. The abundance of each isotope in nature also contributes to the average atomic mass.

To effectively learn and master this concept, practice is key. Worksheets designed for calculating average atomic mass are invaluable tools for students. These worksheets typically present problems involving different isotopes of an element, their respective masses (usually in atomic mass units, amu), and their percent abundance. Students then need to apply a specific formula to arrive at the correct average atomic mass. This process helps solidify the understanding of isotopes, abundance, and their combined effect on the element’s overall atomic mass.

Mastering the calculation of average atomic mass has significant implications beyond the classroom. It allows us to understand the composition of materials, predict chemical reactions, and perform accurate quantitative analysis. It’s a foundational skill that supports more advanced concepts in stoichiometry, chemical kinetics, and nuclear chemistry. Therefore, dedicating time and effort to practice these calculations is a worthwhile investment in your chemical education.

Now, let’s dive into some examples and their solutions. Remember the formula for calculating average atomic mass:

**Average Atomic Mass = (Mass of Isotope 1 × % Abundance of Isotope 1) + (Mass of Isotope 2 × % Abundance of Isotope 2) + …**

Where the percentage abundance must be converted to a decimal before multiplication (e.g., 75% becomes 0.75). Let’s consider a typical worksheet, and then we’ll present the answers in a clear and accessible format.

Calculating Average Atomic Mass Worksheet: Example Problems

Below are some example problems that might be found in a Calculating Average Atomic Mass worksheet. These are representative examples to illustrate the types of calculations you might encounter.

Example Problems:

  1. Element X has two isotopes. Isotope 1 has a mass of 10.0 amu and an abundance of 20%. Isotope 2 has a mass of 12.0 amu and an abundance of 80%. Calculate the average atomic mass of element X.
  2. Magnesium has three isotopes: Magnesium-24 (23.985 amu, 78.99%), Magnesium-25 (24.986 amu, 10.00%), and Magnesium-26 (25.983 amu, 11.01%). Calculate the average atomic mass of Magnesium.
  3. Gallium has two naturally occurring isotopes: Gallium-69 (68.9256 amu) and Gallium-71 (70.9247 amu). If the average atomic mass of Gallium is 69.723 amu, what is the percent abundance of each isotope? (This one requires slightly more algebra!)
  4. Copper has two isotopes: Copper-63, which has a mass of 62.9296 amu and a relative abundance of 69.17%, and Copper-65. The average atomic mass of copper is 63.546 amu. Calculate the mass of Copper-65.

These problems showcase the different variations of average atomic mass calculations. Some require direct application of the formula, while others involve solving for an unknown abundance or mass. Practicing these variations will significantly improve your understanding of the concept.

Answers to Example Problems

Below are the answers to the example problems listed above. Remember to show your work when completing these types of problems to ensure full credit and demonstrate your understanding of the process.

  1. Element X:
    • Average Atomic Mass = (10.0 amu * 0.20) + (12.0 amu * 0.80) = 2.0 amu + 9.6 amu = 11.6 amu
  2. Magnesium:
    • Average Atomic Mass = (23.985 amu * 0.7899) + (24.986 amu * 0.1000) + (25.983 amu * 0.1101) = 18.946 amu + 2.499 amu + 2.861 amu = 24.306 amu
  3. Gallium:
    • Let x = abundance of Gallium-69. Then (1-x) = abundance of Gallium-71.
    • 69.723 amu = (68.9256 amu * x) + (70.9247 amu * (1-x))
    • 69.723 = 68.9256x + 70.9247 – 70.9247x
    • -1.2017 = -1.9991x
    • x = 0.6011
    • Gallium-69 abundance = 0.6011 * 100% = 60.11%
    • Gallium-71 abundance = (1-0.6011) * 100% = 39.89%
  4. Copper:
    • Let m = mass of Copper-65.
    • 63.546 amu = (62.9296 amu * 0.6917) + (m * 0.3083)
    • 63.546 amu = 43.523 amu + 0.3083m
    • 20.023 amu = 0.3083m
    • m = 64.944 amu

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