Question

A naturally occurring sample of lithium contains 7.42% of ⁶Li and 92.58% of ⁷Li. The relative atomic mass of ⁶Li is 6.015 and that of ⁷Li is 7.016.Calculate the atomic mass of a naturally occurring sample of lithium.

Answer 1

Percentage composition of ⁶Li = 7.42% 

Relative atomic mass of ⁶Li = 6.015 

Percentage composition of the ⁷Li = 98.58% 

Relative atomic mass = 7.016 

Atomic mass = (percentage isotopic composition × relative mass of ⁶Li) + (percentage isotopic composition × relative mass of ⁷Li) 

Atomic mass = (0.0742 × 6.015) + (0.9258 × 7.016) 

= 0.446 + 6.49 

= 6.936u