The Mole and Stoichiometry

Question

Element A has two isotopes. The first isotope is present 11.06% of the time and has a mass of 137.77. The second isotope has a mass of 122.45. Calculate the atomic mass of element A. (To two decimals places)

Answer 1

The atomic mass is a weighted average of the different isotopes.

Isotope 137.77 is 11.06%

Isotope 122.45 is 100 - 11.06% = 88.94%

(0.1106 x137.77) + (0.8894 x 122.45) = x

15.237 + 108.91 = x

x = 125.15 amu