Question

Variations in average atomic mass may be observed for elements obtained from different sources. Lithium provides an example of this. The isotopic composition of lithium from naturally occurring minerals is 7.5% 6Li and 92.5% 7Li, which have masses of 6.01512 amu and 7.01600 amu, respectively. A commercial source of lithium, recycled from a military source, was 3.75% 6Li (and the rest 7Li). Calculate the average atomic mass values for each of these two sources.

Answer 1

Answer:

Average atomic mass of lithium from source-I= 7.291734 amu

Average atomic mass of lithium from source-II= 6.78467 amu

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) Average atomic mass of lithium from source-I

Atomic mass of Li-6=6.01512 amu

Fractional abundance of Li-6= 7.5% =0.075

Atomic mass of Li-7=7.01600 amu

Fractional abundance of Li-7= 92.5% =0.975

Average atomic mass : $6.01512 amu\times 0.075+7.01600 amu\times 0.975$

Average atomic mass of lithium from source-I= 7.291734 amu

2) Average atomic mass of lithium from source-II

Atomic mass of Li-6=6.01512 amu

Fractional abundance of Li-6= 3.75% =0.0375

Atomic mass of Li-7=7.01600 amu

Fractional abundance of Li-7= 100%- 3.75% =96.25%=0.9625

Average atomic mass : $6.01512 amu\times 0.0375+7.01600 amu\times 0.9625$

Average atomic mass of lithium from source-II= 6.78467 amu