Consider an element Z that has two naturally occurring isotopes with the following percent abundances: the isotope with a mass number of 19.0 is 55.0% abundance and the isotope with a mass number of 21.0 is 45.0 % abundant. What is the atomic mass for element Z

The atomic mass for element Z is 19.9

Explanation:

Mass number and relative abundance of the first isotope is 19 and 55% respectively.

Mass number and relative abundance of the second isotope is 21 and 45% respectively.

Mass number of element Z = (0.55*19) + (0.45*21) = 10.45 + 9.45 = 19.90