Question

Fill in the blanks: The formula unit mass of ${\mathrm{Ca}}_3{\left({\mathrm{PO}}_4\right)}_2$ is..................…

Answer 1

Step 1: Analyse atomic weights of elements in Calcium Phosphate ( ${\mathrm{Ca}}_3{\left({\mathrm{PO}}_4\right)}_2$)

The atomic mass of Calcium (Ca)$=40.7\mathrm{u}$

So, for three Ca atoms, atomic mass = $3\times 40.7=120.21\mathrm{u}$

The atomic mass of Phosphorus (P) $=30.9\mathrm{u}$

So, for two P atoms, atomic mass = $2\times 30.9=61.8\mathrm{u}$

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

So, for eight O atoms, atomic mass = $8\times 16=128\mathrm{u}$

Step 2: Analyse formula unit mass

Formula unit mass for ${\mathrm{Ca}}_3{\left({\mathrm{PO}}_4\right)}_2$ is:

${\mathrm{Ca}}_3{\left({\mathrm{PO}}_4\right)}_2=(3\times \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{Ca})+(2\times \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{P})+(8\times \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{O})$

$$\begin{gathered} =120.21+61.8+128 \\ =309.9\mathrm{u}\cong 310\mathrm{u} \end{gathered}$$

Hence, the formula unit mass for ${\mathrm{Ca}}_3{\left({\mathrm{PO}}_4\right)}_2$ is $310\mathrm{u}$.