To make a solution of pH = 12, the amount of NaOH dissolved in one liter of the solution should be

pH = 12  ; ${\mathrm{P}}^{\mathrm{OH}} =14-12= 2$

or      $\left[{\mathrm{OH}}^{-}\right]={10}^{-2} \mathrm{M}$

or     $\left[\mathrm{NaOH}\right]={10}^{-2} \mathrm{moles}/\mathrm{lit}$                           

$$\begin{gathered} \mathrm{No}. \mathrm{moles}\left(\mathrm{n}\right) =\frac{\mathrm{w}}{\mathrm{Gmw}} \\ \mathrm{W}={10}^{-2}\times 40 = 0.4 \mathrm{g} {\mathrm{L}}^{-1} \end{gathered}$$