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4 January, 17:24

An element has two naturally-occurring isotopes. The mass numbers of these isotopes are 125 amu and 127 amu, with natural abundances of 45% and 55%, respectively. Calculate its average atomic mass. Report your answer to 1 decimal place

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  1. 4 January, 20:38
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    The average atomic mass is 126.1

    Explanation:

    To obtain the atomic mass of the isotope we use this formula;

    (% composition of isotope 1 * mass number) + (% composition of isotope 2 * mass number)

    1. Equate the percentage composition with the mass number of each isotope

    45/100 * 125 = 56.25

    55/100 * 127 = 69.85

    2. Add both answers together you get,

    56.25 + 69.85 = 126.1.

    The average atomic mass is therefore 126.1
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