Argon has 3 isotopes mass of 36,38,40. Relative abundances & atomic masses: 0.0337% , 35.978 amu; 0.063% , 37.963 amu & 99.600%, 39.962. Calculate the average atomic mass.

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Argon has three isotopes. The abundance of the isotope with mass 35.978 amu is 0.0337%, the abundance of the isotope with mass 37.963 amu is 0.063% and the isotope with mass 39.962 amu has an abundance of 99.6%.

The mean atomic mass is the weighted average of the mass of each isotope. This gives: (0.0337*35.978 + 0.063*37.963 + 99.6*39.962)/100 = 3983.81/100 = 39.8381

The average mass of argon is 39.8381 amu.

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