The atomic mass of chlorine is listed as 35.454 amu on the periodic table. It has two naturally occurring isotopes, CI-35 and CI-37. What are the approximate percentage of each?

Answer 1

If you want REAL numbers you can get them from www.webelements.com.
If you want to calculate them, you need to know the masses of the 35 and the 37.
x = fraction of the 35.
1-x = fraction of the 37

x(mass 35) + (1-x)(mass 37) = 35.454.