Half life is independent of the concentration of A. After 10 min volume of N, gas is 10 L and after complete reaction is 50 L. Hence, the rate constant is

It follows first order kinetics since T50 is independent of concentration.                              A           ⟶                 PhCl       +          N2 t = 0                      a                                   0                          0 t = 10 min       (a - x)                              x                           x complete                0                                  a                           aHence, x = 10 L, a = 50 L∴k=2.30310log⁡5040=2.30310log⁡1.25min−1