Question

Lithium from naturally occurring minerals has isotopic compositions of 7.5% 6Li and 92.5% 7Li, with masses of 6.01512 amu and 7.01600 amu, respectively. What is the average atomic mass of lithium from these sources?

a) 6.778 amu

b) 6.940 amu

c) 7.021 amu

d) 6.758 amu

Answer 1

The average atomic mass of lithium, given the isotopic composition 92.5% ^7Li and 7.5% ^6Li, is approximately 6.940 amu which corresponds to option b).

Step-by-step explanation:

The average atomic mass of an element can be determined by considering the mass and relative abundance of each isotope. For lithium with isotopic compositions 92.5% 7Li (mass 7.01600 amu) and 7.5% 6Li (mass 6.01512 amu), the average atomic mass is calculated as follows:

• Multiply the mass of each isotope by its relative abundance (expressed as a decimal).
• Add the results together to get the average atomic mass.

Calculation for lithium:

(7.01600 amu × 0.925) + (6.01512 amu × 0.075) = 6.4668 amu + 0.451134 amu = 6.917934 amu

The average atomic mass of lithium from these sources is approximately 6.917934 amu. Considering significant figures and the options given, this rounds to 6.940 amu, which corresponds to option b).