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Mole Calculations

Calculation of Ar based on Isotopic Abundance

May 8, 2010 By Sean Chua 3 Comments

In GCE A-Levels, many question on Atomic Structure are based on Isotopes, more specifically it will be testing you on Isotopic Abundance.

We all know that Chlorine has a relative atomic mass, Ar of 35.5. How did we get it?

It is based on the Isotopic Abundance Calculation on its 2 isotopes, namely 35Cl (75% abundance) and 37Cl (25% abundance).

Let’s see how we get the value of 35.5 for its Ar.

Ar =  ∑(Isotopic Mass x % Abundance) / 100

Putting the formulae into action, we will have:

Ar of Cl = [(75 x 35) + (25 x 37)] / 100 = 35.5

Got it?

How about trying out a question on your own?

Question:

Given that Lead has four isotopes: 204Pb, 206Pb, 207Pb and 208Pb. Relative abundance is 2%, 24%, 22% and 52% respectively. Calculate the relative atomic mass of Lead.

PS: Try it out and leave your answer in the comments section below.

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