Calculating relative atomic mass?

Question

Calculate the relative atomic mass of gallium given that the relative abundance of its two isotopes are: 60.5% of Ga-69 and 39.5% of Ga-71. (Show work)

Answer 1

Atomic mass of gallium 69 = 68.925580 amu
Atomic mass of gallium-71 = 70.924700 amu

Relative atomic mass = (0.605)(68.925580) + (0.395)(70.924700)

Relative atomic mass = 41.699976 + 28.015256 = 69.715232

The relative atomic mass is just the weighted average of the masses of all the isotopes. Since Ga-69 and Ga-71 are by a long shot the most abundant isotopes, this weighted average will be plenty accurate for most purposes.

From the CRC Handbook, the actual abundances of Ga-69 and Ga-71 are 60.1% and 39.9%, respectively, giving a relative atomic mass of 69.72. I suspect your prof is using the "adjusted" percentages to ensure that you're not just copying the answer off a Periodic Table of the elements.