Calculating % Abundance of Br-79 & Br-81

• sjung915
Solving this system will give you the abundance of each isotope.In summary, the conversation involves determining the percentage abundance of two isotopes of naturally occurring bromine, Br-79 and Br-81, given their atomic weights and the overall atomic weight of Br. The correct equation for solving this problem is (x1)(78.9183) + (x2)(80.9163) = 79.904, where x1 and x2 represent the abundance of each isotope. Another equation, x1 + x
sjung915

Homework Statement

Naturally occurring bromine contains two isotopes; Br-79, which has a weight of 78.9183 and Br-81 with a weight of 80.9163. If the atomic weight of Br is 79.904, determine the % abundance of Br-79 and Br-81.

Homework Equations

To solve this equation I try to put the equation as

(x)(78.9183) + (x)(80.9163) = 79.904

The value of x = .499946789

The Attempt at a Solution

The answers were given because this is a practical exam I'm studying for. Can someone point me in the right direction. Something seems to be missing from the equation.

Br-81 49.33%
Br-79 50.66%

Nevermind I found the answer.. Thank you

For the record:

sjung915 said:
(x)(78.9183) + (x)(80.9163) = 79.904

is wrong, as abundances of both isotopes are different, not identical - so you need two variables. The correct equation is

(x1)(78.9183) + (x2)(80.9163) = 79.904

and you need another equation:

x1 + x2=1

1. How is % abundance of Br-79 and Br-81 calculated?

The % abundance of Br-79 and Br-81 is calculated by dividing the number of atoms of each isotope by the total number of atoms of all isotopes of bromine present in a sample and then multiplying by 100 to get a percentage.

2. What is the formula for calculating % abundance?

The formula for calculating % abundance is (% of isotope x total number of atoms of that isotope) / (total number of atoms of all isotopes) x 100.

3. How do we determine the number of atoms of each isotope in a sample?

The number of atoms of each isotope in a sample can be determined through experimental methods such as mass spectrometry or by using the atomic weight and % abundance of each isotope to calculate the number of atoms present.

4. How is the atomic weight of each isotope taken into account when calculating % abundance?

The atomic weight of each isotope is multiplied by its % abundance to determine the contribution of that isotope to the total atomic weight of the element. This is then used to calculate the % abundance of each isotope in the sample.

5. Can % abundance change over time?

Yes, % abundance can change over time due to natural processes such as radioactive decay or human activities like nuclear reactions. However, the rate of change is usually very small and can be measured and accounted for in scientific calculations.

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