Question

Lithium, is used in dry cells and storage batteries and in high temperature lubricants, it has two naturally occurring isotopes, 6Li and 7Li. Calculate the atomic mass of lithium.

Answer 1

Answer: The average atomic mass of lithium is 6.9241 u.

Explanation:

Average atomic mass of an element is defined as the sum of masses of each isotope each multiplied by their natural fractional abundance.

Formula used to calculate average atomic mass follows:

$\text{Average atomic mass }=\sum_{i=1}^n\text{(Atomic mass of an isotopes)}_i\times \text{(Fractional abundance})_i$ ....(1)

• For $_3^6\textrm{Li}$ isotope:

Mass of $_3^6\textrm{Li}$ isotope = 6 u

Percentage abundance of $_3^6\textrm{Li}$ isotope = 7.59 %

Fractional abundance of $_3^6\textrm{Li}$ isotope = 0.0759

• For $_3^7\textrm{Li}$ isotope:

Mass of $_3^7\textrm{Li}$ isotope = 7 u

Percentage abundance of $_3^7\textrm{Li}$ isotope = 92.41%

Fractional abundance of $_3^7\textrm{Li}$ isotope = 0.9241

Putting values in equation 1, we get:

$\text{Average atomic mass of Lithium}=[(6\times 0.0759)+(7\times 0.9241)]$

$\text{Average atomic mass of Lithium}=6.9241u$

Hence, the average atomic mass of lithium is 6.9241 u.