Why does a #23*g# mass of calcium hydroxide dissolved in a #400*mL# volume of water, have #[HO^-]-=1.55*mol*L^-1#?
It seems that you have grasped the concept of stoichiometry:
Each mole of calcium hydroxide clearly gives 2 moles of hydroxide anion upon dissolution, simply because of the formulation of calcium hydroxide. Is this clear?
To put it another way, the given solution is