Assertion (A): $\text{Vapour density = }\frac{\text{Density of gas}}{\text{Density of hydrogen gas}}$

  Reason (R): Vapour density is equal to $\text{2\hspace{0.17em}}\times \text{\hspace{0.17em}molecular\hspace{0.17em}mass}$

$\text{Vapour density = }\frac{\text{Density of gas}}{\text{Density of hydrogen gas}}$

Actually, the correct relation between vapor density and molecular mass of any compound is shown below. 

$\text{Molecular\hspace{0.17em}mass =\hspace{0.17em}2}\times \text{Vapor\hspace{0.17em}density }$