# Need help on Isotopes

## 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.

## The Attempt at a Solution

phyzguy
The molar mass is the average mass of all of the isotopes. If 57% is 121, what must the other one isotope be in order for the average to be 121.84?

epenguin
Homework Helper
Gold Member
Yes, it's got to be a small whole number, now just over 40% if this other isotope adds 0.84 to the average, I think I can work out what it must be in my head, and if not just try 1, 2, 3,... you will never have to go very far.

Yes, it's got to be a small whole number, now just over 40% if this other isotope adds 0.84 to the average, I think I can work out what it must be in my head, and if not just try 1, 2, 3,... you will never have to go very far.

I don't follow your reasoning; the number must be greater than 121 for the average to be 0.84 higher with a lower abundance. I may just be misunderstanding, so sorry=]

allie, I'll assume you know how to take the average mass of two isotopes, but you may not have looked at the full equation for the mass; maybe this'll help :)

in this case $$\frac{(121 \times 57.3) + (M \times (100-57.3))}{100} = 121.84$$