Question

(ii) Lithium exists in nature in the form of two isotopes , Li -6 and Li-7 With atomic masses 6.051U and 7.016U and the % 8.24 and 91.76 respectively. Calculate average atomic mass of Lithium.​

Answer 1

Given:

Li - 6

Atomic mass: 6.051U

Percentage abundance: 8.24%

Li - 7

Atomic mass: 7.016U

Percentage abundance: 91.76%

To find:

Average atomic mass of Li isotopes

Solution:

In order to find out the average mass of the isotopes we should be aware about the equation we are going to use:

Average Atomic mass = Σ​( Isotope mass × Natural Abundance)

Li-6

isotope mass × natural abundance

= 6.051 × 0.0824

= 0.498

Li-7

Isotope mass × Natural abundance

=7.016 × .9176

= 6.437

∴Average atomic mass = ( 6.051 × 0.0824 ) + ( 7.016 × .9176 )

                                       = ( 0.498 ) + ( 6.437 )

                                       = 6.935

∴ Average atomic mass of Lithium isotopes is 6.935U