Chlorine has two isotopes with masses 34.98 u and 36.98 u occurring in abundances of 75.4% and 24.6% respectively. What will the average mass of chlorine be?

34 38 32 35

34

Evidently, the average mass will be somewhere in between 34 and 36 which leaves us with only one option - 35. Alternatively, if you want go by calculations, : Average mass = (34.98 x 75.4 + 36.98 x 24.6)/100