Question

How do I calculate the atomic mass of lithium, which occurs as two isotopes: lithium-6 with a mass of 6.015122 amu and 7.30% natural abundance, and lithium-7 with a mass of 7.016004 amu and 92.70% natural abundance?

Answer 1

To calculate the atomic mass of lithium, you need to consider the contributions from its two isotopes, lithium-6 and lithium-7, taking into account their natural abundances.

Here's the calculation:

1. Multiply the mass of each isotope by its natural abundance, expressed as a decimal. For lithium-6: Mass = 6.015122 amu Abundance = 7.30% = 0.0730 (as a decimal)

   For lithium-7: Mass = 7.016004 amu Abundance = 92.70% = 0.9270 (as a decimal)

   Multiply the mass of lithium-6 by its abundance: (6.015122 amu) x (0.0730) = 0.438729746 amu Multiply the mass of lithium-7 by its abundance: (7.016004 amu) x (0.9270) = 6.493383108 amu

2. Add the results together to obtain the average atomic mass: Average atomic mass = (0.438729746 amu) + (6.493383108 amu) = 6.932112854 amu

Therefore, the atomic mass of lithium, accounting for its isotopic abundances, is approximately 6.932112854 amu.