Assuming a theoretical setup where only the anodic removal of OH ^- affects the solution, and no change in volume, the time required to obtain a solution of pH = 4 by electrolysis of 100 mL of 0.1 M NaOH (using current 0.5 A ) will be:

Step-by-step solution

Initial moles of OH^- = M V = 0.1 0.1 = 0.01 mol Target pH = 4 [H^+] = 10⁻⁴ M, meaning solution is acidic practically all OH^- must be removed. Moles of OH^- to be removed 0.01 mol Anodic reaction: 4OH^- O₂ + 2H₂O + 4e^- 1 mole of OH^- releases 1 mole of electrons _e = 0.01 mol Q = n_e F = 0.01 96500 = 965 C t = Q I = 965 0.5 = 1930 s = 1.93 10^3 s