We’re being asked to **calculate the average atomic mass** of Silicon** (Si)**. We can use the following equation:

$\overline{){\mathbf{Atomic}}{\mathbf{}}{\mathbf{mass}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}{\left[\mathbf{mass}\mathbf{}\mathbf{\times}\mathbf{}\mathbf{f}\mathbf{.}\mathbf{a}\mathbf{.}\right]}_{{\mathbf{isotope}}_{\mathbf{1}}}{\mathbf{}}{\mathbf{+}}{\mathbf{}}{\left[\mathbf{mass}\mathbf{}\mathbf{\times}\mathbf{}\mathbf{f}\mathbf{.}\mathbf{a}\mathbf{.}\right]}_{{\mathbf{isotope}}_{\mathbf{2}}}{\mathbf{+}}{\mathbf{}}{\left[\mathbf{mass}\mathbf{}\mathbf{\times}\mathbf{}\mathbf{f}\mathbf{.}\mathbf{a}\mathbf{.}\right]}_{{\mathbf{isotope}}_{\mathbf{3}}}}$

where atomic mass = average atomic mass of the element and f.a. = fractional abundance of the isotope. To get * f.a.*, we simply need to divide the given percent abundance by 100.

What is the average atomic mass of silicon? Given the following data, calculate the average atomic mass of silicon.

isotope = Si-28

amu =27.9769

abundance(%) =92.18

isotope = Si-29

amu =28.9765

abundance(%) =4.71

isotope =Si-30

amu =29.9738

abundance(%) =3.12

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