Question

What is the average atomic mass of bromine? One isotope of bromine has an atomic mass of $ 78.92amu $ and a relative abundance of $ 50.69%. $ The other major isotope of bromine has an atomic mass of $ 80.92amu $ and a relative abundance of $ 49.31%. $

Answer 1

Hint: In order to calculate the molar mass of Bromine, we must first know what a molar mass is. Molar mass can also be referred to as the Molecular weight. Molar mass of the chemical compound is said to be the mass of one mole of the compound. The average atomic mass of an element is the sum of the masses of its isotopes. The protons and neutrons of the nucleus account for nearly all of the total mass of the atom.

Complete step by step solution:
Isotopes are those atoms of a chemical element which have the same atomic number, same position in the periodic table and nearly the same chemical properties but they differ in the mass number and physical properties. The elements have the same number of electrons and protons but it is the number of neutrons that differs.
The atomic mass of an isotope and the relative isotopic mass refers to a certain specific isotope of an element. Because substances are not isotopically pure, it is convenient to use the elemental atomic mass which is the average atomic mass of an element, weighted by the abundance of the isotope.
We need to follow the following steps to calculate the molar mass.
First, we have to identify the formula of a given compound or molecule.
Then determine the number of moles of atoms present in the given compound or molecule.
We have to multiply the mass of each atom with the number of moles of the atom.
This has to be done for every element present in the compound.
Finally add all these values.
The average mass is the mass of each isotope multiplied by its percentage. Thus,
$ Average\text{ }mass=[ 50.69\%( 78.92u ) ]+[ 49.31\%( 80.92u ) ]=[ ( 40.00\text{ }u )+( 39.90\text{ }u ) ]=79.91\text{ }u $
The second method is easier, but it doesn't explain intuitively why the answer is the average atomic mass.

Note:
We must always remember that molar mass is different from molecular mass. Isotopes are those which have the same atomic number but different mass number. Isobars are those atoms which have the same mass number but different atomic numbers. Isotones are those which have the same neutron number but different proton number. It should not be confused with each other.