Rubidium has two naturally occurring isotopes with masses 84.91 amu and 86.91 amu and has an atomic mass of 85.47 amu. part a which mass spectrum is most likely to correspond to a naturally occurring sample of rubidium?

Answer is: mass spectrum of isotope with mass 84.91 amu.

Ar₁(Rb) = 84.91 amu; the average atomic mass of isotope 1.

Ar₂(Rb) =86.91; the average atomic mass of isotope 2.

ω₁(Rb) = ?.

ω₂(Rb) = ?.

ω(Rb) = ω₁ + ω₂.

Ar(Rb) = 85.47 amu; average atomic mass.

Ar(Rb) = Ar₁(Rb) · ω₁(Rb) + Ar₂(Rb) · ω₂(Rb).

85.47 amu = 84.91 amu · ω ₁ + 86.91 amu · ω₂.

ω ₁ = 100% - ω₂.

85.47 amu = 84.91 amu · (1 - ω₂) + 86.91 amu · ω₂.

2ω₂ = 0.56.

ω₂ = 0.28 = 28%.

ω₁ = 72%.

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