9. Hypochlorous acid (HOCI) is 20% more potent a disinfectant than the hypochlorite ion (OCI"). In what pH range should a water treatment plant operate in order to most effectively disinfect water? 10. The pH of the Mississippi River is 8.2. What is the maximum concentration of magnesium ion (Mg2+) in the river? The pks of Mg(OH)2 is 11.25. 12. A confined space has a gas-phase benzene concentration of 0.8 ppmv, just below the OSHA permissible exposure limit (see Problem 5). What mass of styrene is present in a water droplet present in this confined space? Assume the water droplet is 0.150 mL in volume and has achieved equilibrium with the gas phase. The Henry's Law constant for benzene, H, is 240 atm. (This value is reported in Appendix A-2 in your textbook.)