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11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 8X16 and 8X18 in the sample?
Answer :

Let the percentage of 8X16 be ‘a’ and that of 8X18 be ‘100-a’.

As per given data,

16.2u = 16 a / 100 + 18 (100-a) /100

1620 = 16a + 1800 – 18a

1620 = 1800 – 2a

a = 90%

Hence, the percentage of isotope in the sample 8X16 is 90% and that of

8X18 = 100-a = 100- 90=10%