Question

chlorine contains two isotopes CL³⁵ and Cl³⁷ if Cl has average atomic mass 35.5 gram then calculate % abundance of Cl³⁷​

Answer 1

The atomic weight of chlorine is 35.5, so right away we can tell that there will be more Cl35 than Cl37 (if we had 50% of each, they would average at 36 - 35.5 is less than 36, so we have more Cl35)

The percentages of each must add to 100%:

% of Cl35 + % of Cl37 = 100

Now, we have two unknowns so it will be difficult to find a straight-forward answer. Most questions like this will ask you to calculate the atomic weight, but regardless, we know there has to be more Cl35 than Cl37. So from here, its really trial and error.

Because its not 50% each, our next guess will be 75% Cl35 and 25% Cl37

35*75 + 37*25 = 35.5

So your percent abundance for each is 75% for Cl35 and 25% for Cl37.