Calculate the number of moles of magnesium oxide in $10 \mathrm{g}$ of the compound $\mathrm{MgO}$. (Average atomic masses of $\mathrm{Mg}=24$ and $\mathrm{O}=16$)

Calculate the mass of potassium chlorate required to liberate $6.72 {\mathrm{dm}}^3$ of oxygen at STP. Molar mass of ${\mathrm{KClO}}_3$ is $122.5 \mathrm{g} {\mathrm{mol}}^{-1}$.

Calculate the number of atoms of hydrogen present in $5.6 \mathrm{g}$ of urea, $({\mathrm{NH}}_2{)}_2\mathrm{CO}$. Also calculate the number of atoms of $\mathrm{N}, \mathrm{C}$ and $\mathrm{O}$.

Calculate the mass of sulphur dioxide produced by burning $16 \mathrm{g}$ of sulphur in excess of oxygen in contact process. (Average atomic mass : $\mathrm{S}=32 \mathrm{u}, \mathrm{O}=16 \mathrm{u}$)

Explain the need of the term average atomic mass.

Explain molar mass.

Explain mole concept.