What is the average atomic mass of hafnium given the following abundance information on the isotopes? [Hf-176 (mass 176 amu) has an abundance of 5%]
[Hf-177 (mass 177 amu) has an abundance of 19%]
[Hf-178 (mass 178 amu) has an abundance of 27%]
[Hf-179 (mass 179 amu) has an abundance of 14%]
[Hf-180 (mass 180 amu) has an abundance of 35%]