Ar from isotopic abundances
AQA GCSE Chemistry calculation · part of the free Calculation Climb
Not the same as simply reading Ar off the periodic table. The Ar of an element is a weighted average of its isotopes’ masses.
How to do it — step by step
- Multiply each isotope’s mass by its % abundance.
- Add those together.
- Divide by 100.
Worked example
Chlorine: 75% is Cl-35, 25% is Cl-37.
Ar = (35 × 75 + 37 × 25) ÷ 100 = 3550 ÷ 100 = 35.5
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