Aqueous AB, decomposes according to the first order reaction:

${\mathrm{AB}}_2\left(\mathrm{aq}\right)⟶\mathrm{A}\left(\mathrm{g}\right)+2\mathrm{B}\left(l\right)$

After 20 min the volume of A(g) collected during such a reaction is 20 mL, and that collected after a very long time is 40 mL. The rate constant is:

$$\begin{gathered} {\mathrm{AB}}_2\left(\mathrm{aq}\right)\underset{\mathrm{first} \mathrm{order}}{\to } \mathrm{A}\left(\mathrm{g}\right) + 2\mathrm{B}\left(\mathcal{l}\right) \\ t = 0 a 0 0 \\ t = 20 min (a-x) x 2x \\ t=\infty 0 a 2a \end{gathered}$$