An isothermal liquid phase zero order reaction A $\rightarrow$ B (k = 0.5 mol/m$^{3}$ -s) is carried out in a batch reactor. The initial concentration of A is 2 mol/m$^{3}$. At 3 seconds from the start of the reaction, the concentration of A in mol/m$^{3}$ is $\_\_\_\_\_\_$