Suppose that there were two isotopes of Sodium. 28% of the naturally occurring sodium atoms had a mass of 22, and 72% atoms had a mass of 23. What would the average atomic weight of sodium be?

1st isotope of 22u = 28%

2nd istope of 23u = 72%

average atomic mass = 22 x 28/100 + 23 x 72/100

= 6.16 + 16.76

= 22.92

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