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?

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

As per the 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%