Question

Elemental bromine, Br, has an atomic weight of 79.904 amu. There are two stable isotopes of bromine: 79Br has a mass of 78.9183371 amu and 81Br has an atomic mass of 80.9162906 amu. Knowing that the percent abundance of the two adds up to 100.0%, what is the percent abundance of 81Br to three significant figures?

Answer 1

The two isotopes of Bromine are Br-79 and Br-81

The molar mass of Br = 79.904 g/ mol

Let     Br- 79 = X %

Then   Br-81 = ( 100 – X)%

The mass of 79-Br =78.9183371

The mass of 81 –Br =80.9162906

There fore

79.9041 = X% of (78.9183) + (100% -X%) of (80.9163)

   79.9041 = 78.9183 X + 80.9163 - 80.9163X

That is X ( 80.9163 - 78.9183) = 80.9163 – 79.9041

                                    1.9980 X = 1.0122

There fore

X = 1.0122/ 1.9980

= 0.5066

X% = 50.661% Br-79

100% - 50.661% = 49.339% Br-81

50.661% Br-79 and 49.339% Br-81

Therefore the answer

Percent abundance of Br-81 = 49.33%