According to the Arrhenius definition of acids and bases, NaOH is a base because it increases the concentration of hydroxide in solution. We cannot directly calculate the pH of a solution knowing only the hydroxide concentration, so there are two approaches we can take.
The pOH of the solution is -log(0.4) = 0.4 . Since pH + pOH =14 (always), 14-0.4 = 13.6.
Alternatively, we know that [OH] x [H] = 10^-14. We can then take 10^-14 / 0.4 = 2.5 x 10^-13 M. We can plug this in to pH = -log(2.5 x 10^-13) = 13.6
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