# Gallium has an atomic mass of 69.723. The Ga-69 (68.926 amu) is 60.11%. What is the amu of the other isotope? I'd like to know the set up-thought it was (69.723)(0.6011) + (x)(0.3989)? But it isn't giving me the right answer.

##### 1 Answer

The idea behind solving abundance or amu isotope problems is that each isotope contributes a fraction of its atomic mass - its *abundance* - to the average atomic mass.

So, in your particular case,

The set up you used is correct, since the abundance of the second gallium isotope is indeed equal to **1 - 0.6011 = 0.3989**.

If you look up the actual atomic mass of **70.9247 amu**, the exact same result you'd get using this set up, so I'm not sure why you didn't get the right answer.

Finally, the result makes sense because the average atomic mass of gallium is closer to the lighter isotope,