The answers to the problem are shown below the question, but I would like to understand how those answers were found.

A ground water used as a drinking water supply has a pH of 7.0 and a concentration of dissolved arsenic, all present as AsIII, of 100g/L, well above the drinking water standard. The first unit in the treatment plant is prechlorination in a flash mixer - essentially a CFSTR - which has a water residence time of 2 minutes. If chlorine is added at a dose sufficient to maintain a constant concentration of 1.5 mg/L as Cl2 of free chlorine, what is the estimated concentration of dissolved AsIII in the outlet from the flash mixer? How does the answer change for water at pH6 or pH 8? You can assume steady-state operational conditions here. Answers: [As(III)]out=0.14,0.98, and 0.048g/L at pH7,6, and 8 , respectively