We’re being asked to** determine the mass of Si-30****.** We can use the following equation:

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

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.

Silicon has three naturally occurring isotopes (Si-28, Si-29, and Si-30). Masses and natural abundances for two isotopes are listed here.

Isotope | Mass (amu) | Abundance (%) |

Si-28 | 27.9769 | 92.2 |

Si-29 | 28.9765 | 4.67 |

Si-30 | ? | ? |

You may want to reference (Pages 66 - 69) Section 2.8 while completing this problem.

Find the atomic mass of Si-30 .

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