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Competency #5

Answer and Explanation

The correct answer is C.

1. Average atomic mass = (35+37) / 2.
   
   
   This is a simple arithmetic average but yields an answer of 36.0. This does not match the 35.45 given on the periodic table.
2. There must be approximately 25% chlorine-35 and 75% chlorine-37 Average atomic mass = (35 *25 + 37*75)/100.
   
   
   This gives you a value of 36.5, which is even further away from the value on the periodic table.
3. There must be approximately 75% chlorine-35 and 25% chlorine-37 Average atomic mass = (35*75 + 37*25) / 100.
   
   
   The calculation shown above is correct! The value comes out to be 35.5, which is very close to the 35.45 on periodic table.
4. You cannot figure this out with the given information.
   
   
   The information shown in the problem statement is the only information needed to make this calculation.