Posted by **Gil** on Saturday, January 13, 2007 at 7:21pm.

Natural rubidium has an average mass of 85.4678 amu and is composed of isotopes

85Rb (mass = 84.9117 amu) and 87Rb. The ratio of the isotopes 85Rb/87Rb in natural rubidium is 2.591. What is the mass of 87Rb?

Let x be the fraction of the Rb atoms that are Rb85. x/2.591 is the fraction that are Rb87

x + x/2.591 = 1

1.38595 x = 1

x = 0.7215 = fraction that is Rb85

1-x = 0.2784 = fraction that is Rb87

Avg mass = 85.4678 = (0.7215)(84.9117) + (0.2784)M87

Solve for M87

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