WAEC Chemistry 2023 Paper

Concept:The relative atomic mass is the weighted average of the masses of an element's isotopes, based on their relative abundances.Explanation:Isotope $^{65}_{30}\mathrm{X}$ has mass number $65$ and relative abundance $60\% = 0.60$.Its mass contribution $= 65 \times 0.60 = 39.00$.Isotope $^{66}_{30}\mathrm{X}$ has mass number $66$ and relative abundance $40\% = 0.40$.Its mass contribution $= 66 \times 0.40 = 26.40$.Add the contributions to find the relative atomic mass:$A_r = 39.00 + 26.40 = 65.40$.Answer:The relative atomic mass of $X$ is $65.40$.Correct option: D. 65.40.