Question

If an element has two isotopes, what is the atomic mass if one of the isotopes has a mass of 15.000 amu and makes up 5.000% of the mass of the element while the other isotope has a mass of 16.000 amu and make up the remaining 95.000 %?

Answer 1

The atomic mass of element is the weighted average atomic mass of the element with respect to the abundance of the isotopes of that element 
atomic mass is the sum of the products of the mass of isotopes by their percentage abundance 
atomic mass = 15.000 amu x 5.000 % + 16.000 amu x 95.000 % 
                     = 0.7500 + 15.200
atomic mass of element is therefore 15.950