The average atomic mass of element X is 10.025 amu
The average atomic mass of element X = atomic mass of 10 isotope × relative abundance of 10 isotope + atomic mass of 11 isotope × relative abundance of 11 isotope + atomic mass of 12 isotope × relative abundance of 12 isotope
= 10 × 98% + 11 × 1.5 % + 12 × 0.5 %
= 10 × 0.98 + 11 × 0.015 + 12 × 0.005
= 9.8 + 0.165 + 0.06
= 10.025 amu
So, the average atomic mass of element X is 10.025 amu
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