Question

two isotopes of bromine occur naturally. isotope of bromine: bromine - 79, bromine - 81; percent abundance: 50.69 %, 49.31 %; atomic mass: 78.9183, 80.9163. calculate the average atomic mass of bromine to 3 decimal places.

Explanation:

Step1: Recall average atomic - mass formula

The formula for average atomic mass is $Average\ atomic\ mass=\sum_{i}(Relative\ abundance_{i}\times Atomic\ mass_{i})$.

Step2: Convert percentages to decimals

The relative abundance of Br - 79 is $50.69\%=0.5069$ and the relative abundance of Br - 81 is $49.31\% = 0.4931$. The atomic mass of Br - 79 is $78.9183$ and the atomic mass of Br - 81 is $80.9163$.

Step3: Calculate average atomic mass

$Average\ atomic\ mass=(0.5069\times78.9183)+(0.4931\times80.9163)$
$=0.5069\times78.9183 + 0.4931\times80.9163$
$=39.9046 + 40.0854$
$=79.990$

Answer:

$79.990$