Most elements occur naturally as mixtures of isotopes with different relative abundances.
Hence, the relative atomic mass of an element (as seen in The Periodic Table) is the weighted average of the mass number of the isotopes according to their relative abundance.
In the previous blogpost, you have watched the video on Isotopes and how we calculate the relative atomic mass of an element.
Let’s put our knowledge into solving an examination-based question.
Question:
Naturally occurring gallium, Ga, has two isotopes of mass numbers 69 and 71 respectively. Given that the relative atomic mass of Ga to be 69.7, calculate the percentage abundance of each isotopes of gallium.
Suggested Solution:
Let the percentage abundance of gallium-69 be x mol.
The percentage abundance of gallium-71 will then be (100-x) mol.
Relative atomic mass of Ga = 69x + 71(100-x) / 100 = 69.7
69x + 7100 – 71x = 6970
x = 65%
Percentage abundance of gallium-69 = 65%
Percentage abundance of gallium-71 = (100-65) = 35%
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