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Mass Spectroscopy of Elements

AP Chemistry· difficulty 4/5

Element X has two isotopes: 63X^{63}X (mass 62.93 u) and 65X^{65}X (mass 64.93 u). If the average atomic mass is 63.55 u, the natural abundance of 65X^{65}X is closest to

  • A

    20%

  • B

    31%

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  • C

    69%

  • D

    50%

Explanation

Let ff = fraction of 65X^{65}X. Then 62.93(1−f)+64.93f=63.5562.93(1-f) + 64.93f = 63.55, so 2f=0.622f = 0.62, f=0.31f = 0.31 = 31%. (This is copper.)

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