JAMB Chemistry 2014 Paper

Concept:The relative atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes, based on their percentage abundances.Explanation:Convert each percentage abundance to a fraction by dividing by $100$.For $^{28}\mathrm{Si}$: abundance $= \frac{92}{100} = 0.92$.For $^{29}\mathrm{Si}$: abundance $= \frac{5}{100} = 0.05$.For $^{30}\mathrm{Si}$: abundance $= \frac{3}{100} = 0.03$.Multiply each isotopic mass by its fractional abundance.$0.92 \times 28 = 25.76$$0.05 \times 29 = 1.45$$0.03 \times 30 = 0.90$Add the contributions to obtain the relative atomic mass.$25.76 + 1.45 + 0.90 = 28.11$Therefore, the relative atomic mass of the silicon is $28.11$.Answer:$28.11$ (Option C)