Number of ferric ions in $0.8$ g of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ is:

$\left[\mathrm{Fe}=56; \mathrm{O}=16\right]$

The number of calcium ions in $15.5$ g of ${\mathrm{Ca}}_3({\mathrm{PO}}_4{)}_2$ is:

$\left[\mathrm{Ca}=40; \mathrm{P}=31; \mathrm{O}=16\right]$

The number of atoms in $5.6$ g of nitrogen gas (${\mathrm{N}}_2$) is:     

$\left[\mathrm{N}=14\right]$

The number of atoms in $3.55$ g of chlorine (${\mathrm{Cl}}_2$) is:

$\left[\mathrm{Cl}=35.5\right]$

The mass of nitrogen in $3.011\times {10}^{23}$ atoms is: 

$\left[\mathrm{N}=14\right]$

The mass of hydrogen in $3.011\times {10}^{25}$ atoms is

$\left[\mathrm{H}=1\right]$

The mass of $4$ moles of aluminium atoms is:     

$\left[\mathrm{Al}=27\right]$

The number of molecules of sulphur (${\mathrm{S}}_8$ ) present in $16$ g of solid sulphur is:

$\left[\mathrm{S}=32\right]$