Question

example 2
element x has two naturally - occurring isotopes. the isotope with a mass of 10 amu (¹⁰x) has a relative abundance of 80 percent. the isotope with a mass of 11 amu (¹¹x) has a relative abundance of 20 percent. what is the atomic mass and identity of element x?

isotope	% abundance	mass (amu)	mass contribution
	x
total			total:

try these yourself

1. bromine has two naturally - occurring isotopes, bromine - 79 and bromine - 81. the relative abundances of the isotopes are shown below:

isotopes	percent abundance
bromine - 81
total percentage=

a. what is the percentage abundance of bromine - 81? write your answer in the table above (hint: percentage abundance must be equal to 100%)
b. determine the relative atomic mass of bromine.

isotope	% abundance	mass (amu)	mass contribution
bromine - 81	x
total			total:

1. the percentage abundances of silicon are shown below. use the data to determine relative atomic mass of silicon.

Explanation:

Step1: Find bromine - 81 percentage abundance

Since total percentage abundance is 100%, and bromine - 79 has 50.69% abundance. Let the abundance of bromine - 81 be $x$. Then $x=100 - 50.69$.
$x = 49.31\%$

Step2: Determine relative atomic mass of bromine

The mass of bromine - 79 is 79 amu and its abundance is 50.69% or 0.5069 in decimal form. The mass of bromine - 81 is 81 amu and its abundance is 49.31% or 0.4931 in decimal form.
The relative atomic mass $A_r$ is calculated as $A_r=(79\times0.5069)+(81\times0.4931)$.
$A_r = 79\times0.5069+81\times0.4931=39.0451 + 40.9411=79.9862\approx79.99$ amu

Answer:

a. 49.31%
b. 79.99 amu