Question

Element X is found in two forms: 90.0% is an isotope that has a mass of 20.0, and 10.0% is an isotope that has a mass of 22.0. What is the atomic mass of element X?
A. 20.0
B. 20.2
C. 20.8
D. 21.2
E. 21.8
Can anyone explain this one for me?

Answer 1

$M_{X} = \frac{(20.0\cdot 90\%)+(22.0\cdot10\%)}{100\%} = \frac{2020}{100} = 20.2[u]$

answer: B