How do you calculate relative formula mass?

Hint Before solving this question, first we have to understand the term relative formula mass. The relative formula mass of a substance is the total number of relative atomic masses of the atoms in the numbers appeared in the formula.

Complete step by step solution:
The symbol for relative formula mass is ${{M}_{r}}$ . To calculate the value of relative formula mass we have to find out the number of atoms of every element which is present in the given chemical formula.
After this we will add together the relative atomic mass value of all the atoms of every element which is present. Let’s take an example of calcium chloride.
The atomic mass of calcium = 40 amu.
The atomic mass of chlorine = 35.5 amu
The number of atoms of calcium present in calcium chloride i.e. $CaC{{l}_{2}}$ = 1
The number of atoms of chlorine present in calcium chloride i.e. $CaC{{l}_{2}}$ = 2
Formula mass =
\[\begin{align}
& {{M}_{r}}=(1\times 40)+(2\times 35.5) \\
&\implies 111amu \\
\end{align}\]

Additional information:
Molar mass of a chemical compound is calculated by the ratio of mass of the substance in grams to the number of moles of a substance. Molar mass is generally calculated in grams. To calculate the number of atoms first we have to calculate the number of moles of the given element as the number of moles is already given, then the number of atoms will be equal to the product of number of moles and Avogadro’s number.

Note: The relative atomic mass of an element is represented using the symbol ${{A}_{r}}$ . the atoms have next to no mass , this is why we use the relative atomic mass of an atom while calculating the formula mass.