Question

The isotope 121sb makes up 57.21% of naturally occurring antimony. The only other naturally occurring isotope is 123sb. Deduce the relative atomic mass of antimony according to this data. How do I do that?

Answer 1

The ponderate media between both isotopes gives 121.86.

Explanation:

In each 100 `mol` of Sb you have:

`57.21` mol of `_ ^(121)Sb` and `100-57.21=42.79 mol` of `_ ^(123)Sb`

Assuming the mass of the isotopes is integer (they are close to it) we may calculate the corresponding mass of that amount:

`57.21xx121+42.79xx123=12185.58 g`

As this correspond to 100 mol the atomica mass is:

`12185.58/100=121.86 g//(mol)`.

The real value is `121.76 g//(mol)` for comparison.

As you are talking about relative atomic mass, take the units out, so the answer is only 121.86