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

Solution:

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

₈X¹⁸ = 100-a = 100- 90=10%