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## Homework Statement

I have been working on this for ages and can't work it out.

Naturally occurring antimony (Sb) has a molar mass of 121.84 g/mol and contains only two isotopes. One is 121Sb which is 57.3% abundant. What is the mass number of the other isotope of naturally occurring antimony?

I have been through all my text books and can't find the answer, have also googled and I am totally stuck. I need to show how I came to my answer. I don't need to have the question done for me just how to go about working it out.

Thanks in advance for any help.