Chemistry

Problem

A laboratory reference book lists the naturally occurring atomic mass of the element rubidium (Rb) as 85.47 amu. Two isotopes are known:

^{85}\text{Rb}

and

^{87}\text{Rb}

. Assume that no other isotopes exist. (a) Describe qualitatively, without completing a numerical calculation, the algebraic relationship that connects the average atomic mass of rubidium to the individual isotope masses and their percent abundances. (b) Rubidium-87 is radioactive, with a very long half-life. Explain how a gradual, geologically driven decrease in the natural percent abundance of

^{87}\text{Rb}

would affect the tabulated average atomic mass of rubidium over millions of years. (c) Identify one experimental assumption that must be satisfied for the relationship you described in part (a) to yield an accurate average atomic mass for a bulk sample drawn from Earth’s crust. a. Part (a) – Describe the algebraic relationship. b. Part (b) – Explain the long-term trend in the average atomic mass. c. Part (c) – Identify and discuss one underlying assumption.

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