11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes _{8}X^{16} and _{8}X^{18} in the sample?

Let the percentage of _{8}X^{16} be ‘a’ and that of _{8}X^{18} 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 _{8}X^{16} is 90% and that of

_{8}X^{18} = 100-a = 100- 90=10%