The ratio of masses of oxygen and nitrogen in a particular gaseous mixture is 1 : 4. The ratio of number of their molecule is:

The correct option is D 7 : 32
Let mass of oxygen be 'W' g
therefore mass of nitrogen = 4W g
Moles of $O_2=\frac{W}{32}mol$
Moles of $N_2=\frac{4W}{28}mol$
$\therefore$ $\frac{n_{O_2}}{n_{N_2}}=\frac{W}{32}\times \frac{28}{4W}=\frac{7}{32}$
Thus, $\frac{no.ofmoleculesof{O}_2}{no.ofmoleculesof{N}_2}=\frac{7\times N_A}{32\times N_A}=\frac{7}{32}$

Hence the correct option is (d).