Percent Abundance of Br-81 in Bromine

[mugzieee]

Br consists of Br-79 and Br-81. The atomic mass of Br is 79.9. Find the % of Br-81 in bromine.

 

[Steph]

Do you know a formula for calculating the atomic mass from percentages of different isotopes?

Atomic mass is equal, in this case, to:

[(Br-79 percentage abundance/100) x 79] + [(Br-81 percentage abundance/100) x 81]


Because you know the atomic mass and not the abundances, let percentage abundance of Br-81 be X. Then you can write an expression for the percentage abundance of Br-79, and so can calculate the value you need.

 

[thepatientmental]

The percent abundance of Br-81 in bromine can be calculated using the following formula:

% abundance of Br-81 = (atomic mass of Br - atomic mass of Br-79) / (atomic mass of Br-81 - atomic mass of Br-79) * 100

Plugging in the values given in the question, we get:

% abundance of Br-81 = (79.9 - 79) / (81 - 79) * 100
% abundance of Br-81 = 0.9 / 2 * 100
% abundance of Br-81 = 0.45 * 100
% abundance of Br-81 = 45%

Therefore, the percent abundance of Br-81 in bromine is 45%. This means that out of every 100 atoms of bromine, 45 of them will be Br-81 and the remaining 55 will be Br-79.