Answer:
6.9399amu
Step-by-step explanation:
Isotopes are elements with same atomic number but different atomic masses.
To determine the average atomic mass one must multiply the absolute atomic mass (amu) of each isotope times its absolute atomic mass. This will give the weight average contribution of each isotope. The sum of the weight average contributions gives the final weight average atomic mass of the element. The wt. avg. atomic mass is the value posted on the periodic table.
It is recommended that a table be sketched in the following form ...
Isotope | % fractional amt. X Isotopic mass(amu) => Wt. Avg**.
Li-6 7.59% => 0.0759 x 6.015amu => 0.4565
Li-7 92.41% => 0.9241 x 7.016amu => 6.4835
*amu => atomic mass units ∑ Li-6 + Li-7 = 6.9399amu
**Wt.Avg. => weight average contribution of isotope = fractional amt x Isotopic mass(amu)
*** Final Wt. Avg. = ∑Wt.Avg. Contributions (value posted of periodic table)