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

The correct option is B

7 : 32

$\frac{n_{o_2}}{n_{N_2}}=\frac{\frac{\left(m_{o_2}\right)}{\left(M_{o_2}\right)}}{\frac{\left(m_{N_2}\right)}{\left(M_{N_2}\right)}}$
where, $m_{o_2}$ = given mass $O_2,m_{N_2}$ = given mass of $N_2,$
$M_{o_2}$ = molecular mass of $O_2,M_{N_2}$ = molecular mass of $N_2,$
$n_{o_2}$ = number of moles of $O_2,n_{N_2}$ = number of moles of $N_2,$
$=\left[\frac{m_{o_2}}{M_{N_2}}\right]\frac{28}{32}=\frac{1}{4}\times \frac{28}{32}=\frac{7}{32}$