[BenKenobi]

Atomic mass of CL 35 please.

[Hoodat]

Do you mean ³⁵Cl? If you had a sample of pure ³⁵Cl which had no ³⁷Cl, then you could say it had an atomic weight of 35. But that is not the case. The purpose of using the given atomic mass (35.453) is to accurately know the weight of one mole of chlorine. If you needed one mole of chlorine gas (Cl₂) and used only 70 grams, you would come up almost a gram short.