Calculate the number of atoms in $0.4$ mole of nitrogen. (Average atomic mass : $\mathrm{N}=14 \mathrm{u}$)

Calculate the number of atoms in the following, $1.6 \mathrm{g}$ of sulphur. (Average atomic mass : $\mathrm{S}=32 \mathrm{u}$)

$2.0 \mathrm{g}$ of a metal burnt in oxygen gave $3.2 \mathrm{g}$ of its oxide. $1.42 \mathrm{g}$ of the same metal heated in steam gave $2.27$ of its oxide. Which law is verified by these data ?

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 $122.5 \mathrm{g} {\mathrm{mol}}^{-1}$.