What is the atomic weight of an element that has 4 isotopes with abundance 49% - 64X, 28% - 66X , 4% - 67X, 19%  - 68X.

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The atomic weight of an element is the abundance weighted average of its isotopes. Only stable isotopes of the element are considered to calculate the atomic weight. As it is not possible to accurately determine the natural abundance of each isotope there is an inherent inaccuracy while atomic weight is being determined.

Here, the isotope with mass 64X has an abundance of 49%, for the isotope with mass 66X it is 28%, for 67X it is 4% and 19% of the element is the isotope with mass 68X.

This gives the atomic weight as: 64X*0.49 + 66X*0.28 + 67X*0.04 + 68X*0.19 = 65.44X

The atomic weight of the element is 65.44X

It is not possible to state what element X can be.

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