JAMB Chemistry 2015 Paper

Concept:The atomic mass of an element is the weighted average of the masses of its isotopes.Explanation:Let the relative abundance of the isotope of mass number $37$ be $x\%$.Then the relative abundance of the isotope of mass number $35$ is $(100 - x)\%$.The average atomic mass is:$$\frac{35(100 - x) + 37x}{100} = 35.5$$Simplify the equation:$3500 - 35x + 37x = 3550$$3500 + 2x = 3550$$2x = 50$$x = 25$Therefore, the isotope of mass number $37$ has a relative abundance of $25\%$.Answer:$25\%$ (Option B).