The natural abundance of "Br-81"Br-81 is 49.31%49.31%.
This means that 49.31%49.31% of all of the world's bromine "Br"Br exists in the form of "Br-81"Br-81.
As there is only one other isotope of bromine, 100% - 49.31% = 50.69%100%−49.31%=50.69% of it exists in the form of "Br-79"Br-79.
Therefore, the natural abundance of "Br-79"Br-79 is 50.69%50.69%
The atomic mass of bromine is 79.90479.904 "amu"amu.
Rightarrow 79.904⇒79.904 "amu"amu = 50.69% times x + 49.31% times 80.9163=50.69%×x+49.31%×80.9163 "amu"amu
We need to solve for xx, which in this case is the mass of "Br-79"Br-79:
Rightarrow 79.904⇒79.904 "amu"amu = 0.5069 x + 0.4931 times 80.9163=0.5069x+0.4931×80.9163 "amu"amu
Rightarrow 79.904⇒79.904 "amu"amu = 0.5069 x + 39.89982753=0.5069x+39.89982753 "amu"amu
Rightarrow 40.00417247⇒40.00417247 "amu"amu = 0.5069 x=0.5069x
Rightarrow 78.919259164⇒78.919259164 "amu"amu = x=x
therefore x = 78.919 "amu"
Therefore, the mass of "Br-79" is 79.919 "amu".
