Final answer:
Hypochlorous acid (HOCl) is primarily formed when Cl2 gas is added to water during the disinfection of drinking water.
Step-by-step explanation:
When Cl2 gas is added to water during the disinfection of drinking water, hypochlorous acid (HOCl) is primarily formed. This reaction can be represented by the equation: Cl2 (aq) + 2H2O(l) → HOCl(aq) + H3O+ (aq) + Cl¯ (aq). The addition of chlorine gas to water results in the formation of hypochlorous acid, which is a strong oxidant that effectively kills bacteria and other microbes in drinking water supplies and swimming pools.