The time required for 100% completion of a zero order reaction is
(a is the initial concentration and k is the rate constant)
For a zero order reaction,
kt=[A0]−[A]
As the reaction is 100 percent complete, the final concentration of the reactants is zero.
∴[A]=0.
Given, initial concentration is [A]0=a.
∴kt=a−0
t=ak