What is the pH and pOH of a solution that was made by adding 400mL of water to 350mL of 5.0x10^-3 M NaOH solution?

Answer 1

Assuming the solutions are additive in volume, that is 400+360 = 750 so the new concn is 0.0050 x (350/750) = 0.0233 M.
pOH = -log(0.0233) = 2.63
pH + pOH = pKw = 14. Solve for pH.