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Chapter 2: Q24 E (page 120)

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.

Short Answer

Expert verified

Lithium average mass = (0.075 × 6.01512) + (0.925 × 7.01600)

Li avg. mass = (0.4511 + 6.4898) amu

= 6.94 amu

Step by step solution

01

– Determine the abundance

7.5% =\(\frac{{7.5}}{{100}} = 0.075\)

92.5% = \(\frac{{92.5}}{{100}} = 0.925\)

02

– Determine the Average mass

Average mass =\(\sum\limits_i {{{(fractionalabundance \times isotopicmass)}_i}} \)

Given,6Li = 7.5%, Mass = 6Li = 6.01512 amu

7Li = 92.5%, Mass = 7Li = 7.01600 amu.

Lithium average mass = (0.075 × 6.01512) + (0.925 × 7.01600)

Li avg. mass = (0.4511 + 6.4898) amu

= 6.94 amu

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