# 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?

Oct 23, 2017

The ponderate media between both isotopes gives 121.86.

#### Explanation:

In each 100 $m o l$ of Sb you have:

$57.21$ mol of ${_}^{121} S b$ and $100 - 57.21 = 42.79 m o l$ of ${_}^{123} S b$

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

$57.21 \times 121 + 42.79 \times 123 = 12185.58 g$

As this correspond to 100 mol the atomica mass is:

$\frac{12185.58}{100} = 121.86 g / \left(m o l\right)$.

The real value is $121.76 g / \left(m o l\right)$ for comparison.

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