Element X has two isotopes, of atomic masses $69$ and $71$ , and its average atomic mass is $69.8$ . What mass (in g) of the heavier isotope is present in a $34.9 g$ sample of the element? Give the answer to one decimal place.

Numerical answer type, answer in g, tolerance 0.05.

Answer: 14.2 g

Let the atom fraction of the isotope of mass 71 be

f

69 (1 - f) + 71 f = 69.8

, so

f = 0.40

. Moles of atoms

= 34.9/69.8 = 0.50

. Heavy atoms

= 0.40 \times 0.50 = 0.20

mol, mass

= 0.20 \times 71 = 14.2 g

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