We’re being asked to **to determine the mass of Br-79****.** We can use the following equation:

$\overline{){\mathbf{Atomic}}{\mathbf{}}{\mathbf{Mass}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}{\mathbf{[}\mathbf{mass}\mathbf{\times}\mathbf{f}\mathbf{.}\mathbf{a}\mathbf{.}\mathbf{]}}_{\mathbf{isotope}\mathbf{}\mathbf{1}}{\mathbf{}}{\mathbf{+}}{\mathbf{}}{\mathbf{[}\mathbf{mass}\mathbf{\times}\mathbf{f}\mathbf{.}\mathbf{a}\mathbf{.}\mathbf{]}}_{\mathbf{isotope}\mathbf{}\mathbf{2}}}$

where atomic mass = average atomic mass of the element and f.a. = fractional abundance of the isotope. To get ** f.a.**, we simply need to divide the given percent abundance by 100.

Bromine has two naturally occurring isotopes (Br-79 and Br-81) and has an atomic mass of 79.904 amu. The mass of Br-81 is 80.9163 amu, and its natural abundance is 49.31%. Calculate the mass of Br-79.

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