The ratio of masses of oxygen and nitrogen in a particular gaseous...

Let the mass of oxygen be $x$ gm
$∴$ Mass of nitrogen = $4x$ gm
Number of molecules = 6.023 x 10²³ x number of moles
$∴ \text{Ratio} = \dfrac{\text{6.023} \times 1 {0}^{2 3} \times \text{no. of moles of oxygen}}{\text{6.023} \times 1 {0}^{2 3} \times \text{no. of moles of nitrogen}}$
$= \dfrac{\dfrac{{\text{m}}_{{\text{O}}_{2}}}{{\text{M}}_{{\text{O}}_{2}}}}{\dfrac{{\text{m}}_{{\text{N}}_{2}}}{{\text{M}}_{{\text{N}}_{2}}}} = \dfrac{\dfrac{x}{3 2}}{\dfrac{4 x}{2 8}}$
$= \dfrac{7}{3 2} = \text{7 : 32}$