Question

Element X has 2 following stable isotopes, X-23 (relative abundance 41.9%) and X-25 (relative abundance is [100-41.9]%). What is the average atomic mass of Element X? Record the numerical answer to the hundredths place (2 places after the decimal).

Answer 1

Further explanation

Isotopes : the same atomic number, but different mass numbers  

Atomic mass is the average atomic mass of all its isotopes  

avg atomic mass = m1.%m1 + m2.%m2....mn.%mn

You did not include the mass of each isotope

Element X :

X-23 = 41.9%

X-25 = 58.1%

The average atomic mass of Element X :

$\tt =0.419\times mass~X-23+0.581\times mass~X-25$