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Chemistry Practice Problems: Atomic Mass Calculations I

[Download the accompanying PDF worksheet.]

Calculate the Average Atomic Mass

  1. Calculate the average atomic mass of an element with two naturally occurring isotopes: 85X (72.15%, 84.9118 amu) and 87X (27.85%, 86.9092 amu).  What is this element?
  2. Chromium has the following isotopic masses and relative abundances. Calculate the average atomic mass of chromium to two decimal places.


Mass Number Iostopic Mass (amu) Percent Abundance
50 49.9461 4.35%
52 51.9405 83.79%
53 52.9407 9.50%
54 53.9389 2.36%


[Want more practice problems? Check out Atomic Mass Practice Problems Part II.]



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