2. Lithium-6 is 4% abundant and lithium-7 is 96% abundant. What is the average mass of lithium?

Average atomic mass = 6.96 amu.

Explanation:

Abundance of Li-6 = 4%

Abundance of Li-7 = 96%

Atomic mass of Li-6 = 6 amu

Atomic mass of Li-7 = 7 amu

Average atomic mass = ?

Solution:

Average atomic mass = (abundance of 1st isotope * its atomic mass) + (abundance of 2nd isotope * its atomic mass) / 100

Average atomic mass = (4*6) + (96*7) / 100

Average atomic mass = 24 + 672 / 100

Average atomic mass = 696 / 100

Average atomic mass = 6.96 amu.