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The estimated atomic of silicon is 28.082 a.m.u.

Explanation:

The estimated atomic mass of silicon (), in a.m.u., is equal to the weighted average of atomic masses of silicon available in nature and in terms of their abundances:

(1)

Where:

, , - Relative abundance of each isotope, no unit.

, , - Atomic mass of each isotope, in a.m.u.

If we know that , , , , and , then the estimated atomic mass of silicon is:

The estimated atomic of silicon is 28.082 a.m.u.

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)