Answer
Mass of Si-30 = 30.099
Natural abundance of Si-30 = 3.13%
Work Step by Step
1. The sum of the natural abundances must be equal to 100%.
$$92.2\% + 4.67\% + x = 100\%$$ $$x = 100\% - 92.2\% - 4.67\% = 3.13 \%$$
2. According to the periodic table, the atomic mass of Silicon is equal to 28.09 u. Plug that value into the formula, and calculate the mass of Si-30
$$28.09 = (27.9769 \times 0.922) + (28.9765 \times 0.0467) + (m\times 0.0313)$$ $$28.09 = 27.1479 + 0.0313m$$ $$28.09 - 27.1479 = 0.0313m$$ $$\frac{0.9421}{0.0313} = m$$ $$m=30.099$$
