Lithium-6 has a mass of 6.0151 amu and lithium-7 has a mass of 7.0160 amu. The relative abundance of Li-6 is 7.42% and the relative abundance of Li-7 is 92.58%. Based on this data alone, calculate the average atomic mass for lithium to the correct number of significant digits.

Average atomic mass is the weighted average atomic masses with regard to the relative abundance of the isotopes

average atomic mass of Li = relative abundance of Li-6 x mass of Li-6 + relative abundance of Li-7 x mass of Li-7

average atomic mass of Li = (7.42% x 6.0151 a. m. u) + (92.58% x 7.0160 a. m. u)

= 0.446 + 6.495

= 6.941 amu

average atomic mass of Li is 6.941 amu