In two moles of acetaldehyde $\left({\mathrm{CH}}_3\mathrm{CHO}\right)$ calculate the following
a. Number of moles of carbon
b. Number of moles of hydrogen
c. Number of moles of oxygen 
d. Number of molecules of acetaldehyde

Calculate the number of moles of magnesium oxide, $\mathrm{MgO}$ in $80 \mathrm{g}$ of the compound. (Average atomic masses of $\mathrm{Mg}=24$ and $\mathrm{O}=16$)

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.