Question

A water supply contains 0.5 mg/L ammonia (as nitrogen). Estimate the Cl2 dose in mg/L that would be required to reach breakpoint if no other chlorine-demanding substances are present.

Answer 1

Breakpoint - when chloroamines breakdown and converted to N gas which leaves the system.

when we see the reaction between ammonia and Cl₂ we see that 3 moles of Cl₂ is required to Oxidise 2moles of NH₃ to N₂ .

that means 213gram (mass of three moles of Cl2) is required for reacting with 34 gram of ammonia (2moles)

now we know one mole of ammonia weighs = 17 gram

i.e nitrogen in 17gram ammonia = 14 gram

therefore nitrogen in 34 gram ammonia= (14/17)*34 = 28gram nitrogen

In the question we are given water supply contain 0.5 milligram of ammonia per litre of water

that means it contains (0.5)(14/17) milligram of nitrogen per litre of water i.e 0.411mg/L

we calculate above that 7.6 gram Cl₂ is required per gram of nitrogen

i.e 7.6 mg og Cl₂ per mg of nitrogen

therefore Cl₂ required for 0.411mg/L of nitrogen = (7.6)*(0.411)= 3.12mg/ltr