Question

The average atomic mass of sample of an element X is 16.2 u. What are the percentages of isotopes 16/8X  and 18/8X  in the sample?

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Answer 1

Let % of 16 mass be 'x'

so % of 18 mass will be 100-x

so now

16.2 = [16x + 18(100-x) ]÷100

solving further

1620= 16x + 1800 - 18x

2x = 180

x = 90

so abundace of 16 mass will be 90% and 18 mass will be 10%