Answer: To find the atomic mass of chlorine, the atomic mass of each isotope is multiplied by the relative abundance (the percent abundance in decimal form) and then the individual masses are added together. 34.96885 x 0.7578 = 26.50.36.96590 x 0.2422 = 8.95. 26.50 + 8.95 = 35.45. The atomic mass of chlorine is 35.45 amu. 2.nswer: To find the atomic mass of chlorine, the atomic mass of each isotope is multiplied by the relative abundance (the percent abundance in decimal form) and then the individual masses are added together. 34.96885 x 0.7578 = 26.50.