Answer:
You determine the weighted averages if the individual isotopic masses.
Procedure:
The atomic mass of Si is the weighted average of the atomic masses of its isotopes.
We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its percent abundance).
The three stable isotopes of Si are Si-28 (27.98 u, 92.22 %), Si-29 (28.98 u, 4.69 %), and Si-30 (29.97 u, 3.09 %).
Set up a table for easy calculation.
0.9222 × 27.89 u = x u
0.0469 × 28.98 u = y u
0.0309 × 29.97 u = z u
TOTAL = (x+y+z) u = atomic mass of Si.
