The rate constant of a zero order reaction is $$2.0 \times {10^{ - 2}}mol\,{L^{ - 1}}{s^{ - 1}}.$$     If the concentration of the reactant after 25 seconds is $$0.5\,M.$$  What is the initial concentration?

Solution :
$$\eqalign{ & {\text{For a zero order reaction}} \cr & {\text{Rate constant}} = k = \frac{{a - x}}{t} \cr & 2 \times {10^{ - 2}} = \frac{{a - 0.5}}{{25}} \cr & a - 0.5 = 0.5 \cr & a = 1.0M \cr} $$