Question

Calculate the formula mass of the following compound:

Aluminium oxide $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$

Answer 1

Step 1: Definition of Formula mass:

1. The formula mass of a compound is the sum of the atomic masses of the elements present in the compound.
2. The unit of formula mass is the same as the atomic mass unit.
3. It is denoted by 'u'.

Step 2: Calculate the formula mass of Aluminium oxide

To calculate the formula mass of Aluminium oxide $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$, add the atomic mass of each atom of the element present.

The atomic mass of $\mathrm{Al}$ = $27\mathrm{u}$

The atomic mass of $\mathrm{O}$ = $16\mathrm{u}$

The formula mass of $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ = $2$ X (Atomic mass of $\mathrm{Al}$) + $3$ X (Atomic mass of $\mathrm{O}$)

The formula mass of $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ = $\begin{array}{rcl}\left(2\times 27\right)+\left(3\times 16\right)& =& 54+48=102\mathrm{u}\end{array}$

Hence, the formula mass of Aluminium oxide $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ is $102\mathrm{u}$.