In a water treatment plant, Cl2​ gas is added into water for disinfection. Cl2​ gas hydrolyzes into hypochlorous acid (HOCl) in water, and HOCl undergoes the following reaction:
HOCl⇄H⁺ +OCl⁻pKa=7.54 If the pH of the water is 7 , and the concentration of HOCl is 5mg/L, what is the concentration of OCl⁻in mg/L ?