GATE Civil Engineering (CE) 2011 Solved Paper

Chlorine gas (8mg/L as Cl2) was added to a drinking water sample. If the free chlorine residual and pH was measured to be 2 mg/L (as Cl2) and 7.5, respectively, what is the concentration of residual OCI – ions in the water? Assume that the chlorine gas added to the water is completely converted to HOCI and OCI-. Atomic Weight of Cl: 35.5
Given: OCl^- + H⁺→ HOCl, K = 10^7.5