Calculate the acidity constant of hypochlorous acid - is

Question

Disinfection by chlorine (HOCl/OCl^-) is widely used to reduce pathogens in drinking water.  The ratio of HOCl/OCl^- in water is pH-dependent. 

Because HOCl is a much stronger disinfectant than OCl^-, a typical goal is to achieve [HOCl] = 2 mg/L as Cl₂ (i.e., 2.82´10^-5 M of HOCl) in order to maintain sufficient disinfection efficiency.

Calculate the acidity constant of hypochlorous acid (HOCl) at 25°C, I = 0.0 M.

     HOCl = H⁺  +  OCl^-        K_a = {H⁺}{OCl^-}/{HOCl}  

Is HOCl acid dissociation endothermic or exothermic?

Suppose the source water is at 25°C, I = 0.0 M, and pH = -log{H⁺} = 7.8, how much total chlorine needs to be added to the water in order to achieve [HOCl] = 2 mg/L as Cl₂?