Question

What is amu of 99 % H-1, .2% H-1 and .8% H-3

Answer 1

The average atomic mass of your mixture is 1.03 u .

The average atomic mass of H is the weighted average of the atomic masses of its isotopes.  

We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its % abundance).  

Thus,  

0.99    × 1.01 u = 0.998 u

0.002 × 2.01 u = 0.004 u

0.008 × 3.02 u = 0.024 u

            TOTAL =  1.03   u