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Question

# The atomic masses of two isotopes of an element are 16 and 17 respectively. Their percentage abundances are 25% and 60% respectively and the average atomic mass of the element is 16.90. If the element has 3 isotopes, calculate the atomic mass of the third isotope.

A
17
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B
18
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C
20
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D
15
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Solution

## The correct option is A 18Let the atomic mass of the third isotope be A.The average atomic mass of the element16.90=[25×16]+[60×17]+[15×A]25+60+151690=[25×16]+[60×17]+[15×A]1690=1420+15A15A=270A=18The atomic mass of the third isotope is 18.

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