How to calculate the relative molecular mass?

Relative mass

• The relative molecular mass/relative formula mass is defined as the sum of all the individual atomic masses of all the atoms.

Calculation of relative mass

• The relative formula mass of a substance is the sum of the relative atomic masses of the elements present in a formula unit.
• The symbol for relative formula mass is ${\mathrm{M}}_{\mathrm{r}}$_.
• If the substance is made of simple molecules, this mass may also be called the relative molecular mass.
• Relative mass of an element is calculated by adding the number of protons to the number of neutrons for the specific isotope of the element.
• Example: Calculation of relative mass of ammonia(${\mathrm{NH}}_3$)
• Molecular mass of N= 14$\mathrm{g}{\mathrm{mol}}^{-1}$, Molecular mass of H= 1$\mathrm{g}{\mathrm{mol}}^{-1}$
• Relative mass =$14+(3\times 1)=17\mathrm{g}{\mathrm{mol}}^{-1}$

Therefore, relative mass is calculated by adding together all the atomic masses of elements shown in the formula.