Question:
An element has two isotopes of mass numbers \(65\) and \(67\) with abundances of \(25\%\) and \(75\%\) respectively. Then the average atomic mass of the element is
An element has two isotopes of mass numbers \(65\) and \(67\) with abundances of \(25\%\) and \(75\%\) respectively. Then the average atomic mass of the element is
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Take the abundance-weighted average of the two isotopic masses.
Updated On: Nov 12, 2026
- \(65.5\)
- \(67.5\)
- \(66.5\)
- \(66\)
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The Correct Option is C
Solution and Explanation
Step 1: Understanding the Concept
The average atomic mass is the weighted mean of the isotopic masses, weighted by their percentage abundance.
Step 2: Set Up
Isotope 1: mass \(65\), abundance \(25\%\).
Isotope 2: mass \(67\), abundance \(75\%\).
Step 3: Calculation
\(M = \dfrac{65\times 25 + 67\times 75}{100} = \dfrac{1625 + 5025}{100} = 66.5\)
Final Answer:
The average atomic mass of the element is 66.5. \[ \boxed{\text{(C) }66.5} \]
The average atomic mass is the weighted mean of the isotopic masses, weighted by their percentage abundance.
Step 2: Set Up
Isotope 1: mass \(65\), abundance \(25\%\).
Isotope 2: mass \(67\), abundance \(75\%\).
Step 3: Calculation
\(M = \dfrac{65\times 25 + 67\times 75}{100} = \dfrac{1625 + 5025}{100} = 66.5\)
Final Answer:
The average atomic mass of the element is 66.5. \[ \boxed{\text{(C) }66.5} \]
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