CBSE Class 9 Answered

Solution:

Average atomic mass = 16.2u

Mass of ¹⁶₈X = 16

Mass of ¹⁸₈X = 18

Average atomic mass = (mass of ¹⁶₈X) (percentage) + (mass of ¹⁸₈X) (percentage)

                                                                    100

Let the percentage of isotope 1 with atomic mass 16 be ‘x’

So the percentage of isotope II with atomic mass 18 would be (100 – x)

16.2 = 16x  + 18(100-x)

                       100

Or 1620 = 16x  + 1800 – 18x

 x = 90

Thus, percentage of isotope I (i.e. ¹⁶₈X ) = 90 %

Percentage of isotope II (i.e. ¹⁸₈X ) = 100 – 90 = 10 %