Question

Bromine is a red-orange liquid with an average atomic mass of 79.90 amu. Its name is derived from the Greek word bromos, which means stench. It has two naturally occurring isotopes: Br-79 (78.92 amu) and Br-81 (80.92 amu). What is the abundance of the heavier isotope?

Answer 1

the heavier isotope is Br-81. 

to find the percentage abundance, you can use the following equation:

(exact weight of isotope #1) (abundance of isotope #1) + (exact weight of isotope #2) (abundance of isotope #2) = average atomic weight of the element

so we plug in the values, where x is our percentage abundance

(78.92) (x) + (80.92) (1 - x) = 79.90

we solve for x.

78.92X + 80.92 - 80.92X= 79.90

78.92X - 80.92X = 79.90 - 80.92

-2X= -1.02

X= -1.02 / -2= 0.51 or 51% 

51% represents the abundances of the lighters isotope. to find the heaviest, we just subtract 100 to it. 100-51= 49 %

Answer 2

The abundance of the heavier isotope is 49%

Given that the average atomic mass of bromine is 79.90 amu

Let the mass of the heavier isotope be x

Let the mass of the lighetr isotope be 1 - x

So, we can write;

79.90 = 78.92 ( 1 - x) + 80.92x

79.90 = 78.92 - 78.92x + 80.92x

79.90 - 78.92 = - 78.92x + 80.92x

0.98 = 2x

x = 0.98/2

x = 0.49

Hence, the percentage of the heavier isotope = 0.49 × 100 = 49%

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