Need Help on Isotopes: Find Mass Number of Antimony

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Discussion Overview

The discussion revolves around determining the mass number of the second isotope of naturally occurring antimony (Sb), given its molar mass and the abundance of one of its isotopes. Participants explore the mathematical relationships involved in calculating the average molar mass based on the isotopic abundances.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant states the need to find the mass number of the second isotope based on the average molar mass of antimony and the abundance of the known isotope.
  • Another participant notes that the molar mass is the average of all isotopes and suggests that if 57% is 121, the other isotope must be calculated to achieve the average of 121.84 g/mol.
  • A different participant proposes that the second isotope must be a small whole number and suggests testing various values to find a suitable mass number.
  • One participant expresses confusion about the reasoning that the second isotope must be greater than 121 to raise the average, indicating a potential misunderstanding of the calculations involved.
  • Another participant provides the equation for calculating the average mass of two isotopes, implying that the mass number of the second isotope can be derived from this equation.

Areas of Agreement / Disagreement

Participants express differing views on the reasoning behind the mass number of the second isotope, with some uncertainty about the calculations and assumptions involved. No consensus is reached on the correct approach or answer.

Contextual Notes

Participants do not fully resolve the mathematical steps required to find the mass number of the second isotope, and there are assumptions about the nature of isotopes and their contributions to the average molar mass that remain unexamined.

allie2032
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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 textbooks 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.



Homework Equations





The Attempt at a Solution

 
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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?
 
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.
 
epenguin said:
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
 

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