An element has two naturally-occurring isotopes. The mass numbers of these isotopes are 123 amu and 125 amu, with natural abundances of 95% and 5%, respectively. Calculate its average atomic mass. Report your answer to 1 decimal place.

123.1

Explanation:

The average atomic mass of an element which exhibits isotopy is the average mass of its various isotopes as they occur naturally in any quantity of the element.

Average atomic mass = (95% of 123 amu)+(5% 0f 125 amu)

= (0.95 × 123) + (0.05 × 125)

= 116.85 + 6.25

= 123.1   (to 1 decimal place)

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