Arrange the following in the order of increasing number of atoms 0.2 mole ${\text{NH}}_{\text{3}}$, 0.2 mole ${\text{H}}_{\text{2}}\text{O}$ 0.2 mole CO, 0.2 mole HCl

$\text{0.2 mol of HC}l=0.2\text{\hspace{0.17em}gram\hspace{0.17em}\hspace{0.17em}atoms\hspace{0.17em}of HC}l$

$(0.2\text{\hspace{0.17em}gram\hspace{0.17em}atoms\hspace{0.17em}\hspace{0.17em}of H\hspace{0.17em}}+0.2\text{\hspace{0.17em}gram\hspace{0.17em}atoms\hspace{0.17em}of\hspace{0.17em}C}l)$

$\text{0.2\hspace{0.17em}mol\hspace{0.17em}of CO\hspace{0.17em}}=0.2\text{\hspace{0.17em}gram atoms\hspace{0.17em}of CO\hspace{0.17em}}$

$(0.2\text{\hspace{0.17em}of \hspace{0.17em}gram\hspace{0.17em}atoms of C\hspace{0.17em}\hspace{0.17em}}+0.2\text{\hspace{0.17em}\hspace{0.17em}gram\hspace{0.17em} atoms of\hspace{0.17em}O})$

${\text{0.2\hspace{0.17em}mol\hspace{0.17em}of H}}_{\text{2}}\text{O\hspace{0.17em}=\hspace{0.17em}\hspace{0.17em}}0.2{\text{\hspace{0.17em}gram\hspace{0.17em}atoms\hspace{0.17em}of H}}_{\text{2}}\text{O\hspace{0.17em}}$

$(0.4\text{\hspace{0.17em}gram.\hspace{0.17em}atoms of H +\hspace{0.17em}\hspace{0.17em}0.2\hspace{0.17em}gram.\hspace{0.17em}atoms of 'O')}$

${\text{0.2\hspace{0.17em}mol\hspace{0.17em}of NH}}_3\text{\hspace{0.17em}=\hspace{0.17em}\hspace{0.17em}}0.2{\text{\hspace{0.17em}gram\hspace{0.17em}atoms\hspace{0.17em}of NH}}_3$

$(0.6\text{\hspace{0.17em}gram.\hspace{0.17em}atoms of H +\hspace{0.17em}\hspace{0.17em}0.2\hspace{0.17em}gram.\hspace{0.17em}atoms of 'N')}$

Therefore, the increasing order of number of molecules for given species is

    $\text{HC}l=\text{CO}<{\text{H}}_{\text{2}}\text{O}<{\text{NH}}_{\text{3}}$