Calculating Relative Atomic Mass From Isotopic Abundance

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Isotopes are atoms of the same elements with same number of protons but different number of neutrons.

Many a times, students taking GCE A-Level H2 Chemistry are required to calculate the Relative Atomic Mass of an element with given information on Isotopic Abundance. The abundance of an isotope is the percentage of the isotope found in the naturally occurring element.

It might be given in Atomic Ratio or % Abundance.

Let’s check out an exam-based question that gives you the information in terms on Atomic Ratio.

Example:

The element rhenium consists of two isotopes 185Re and 187Re, in the atomic ratio of 2:3. Calculate the relative atomic mass of rhenium to three significant figures.

Suggested Answer:

Relative Atomic Mass, Ar = [(2 X 185) + ( 3 X 187)] / (2 + 3) = 186 (3 sig fig)

If the information given is in terms of % Abundance, the strategy to solve it is the same – just take the denominator to be 100 since % must add up to 100%.

The best way to learn is to take actions and work on it yourself. Try the question below and leave down your suggested answer.

Quick Check 1:

Chlorine has the following isotopes:

• 35Cl with relative isotopic mass of 34.97 and % abundance of 75.53
• 37Cl wirh relative isotopic mass of 36.97 and % abundance of 24.47

Calculate the relative atomic mass of chlorine.

PS: Try it out! Leave your answers in the comment section below.

Related Articles:

• Calculation of Ar based on Isotopic Abundance
• Isotopes & Isotopic Abundance
• Calculation of Ar based on Isotopic Abundance – Challenging Question
• Sub-Atomic Particles of Atoms
• Key Definitions & Formulae in Atoms, Molecules and Stoichiometry

Reader Interactions

Comments

1. [(34.97 x 75.53) + (36.97 x 24.47)] / (75.53 + 24.47)

   = 35.4 (3 sig fig)

   (:

2. (34.97*75.53) + (36.97*24.47) /100

   =35.4594

   Yeahhh man 🙂

3. Job well done!

   Keep learning & sharing!

   Sean Chua 🙂

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