How many mgL of CrO72 are consumed if the COD of the WasteWa

How many mg/L of CrO₇^-2 are consumed if the COD of the WasteWater sample is found to be 450mg/L ?

Solution

1 mol of Cr2O7 2- consumes 6 mol of electrons to produce 2 mol of Cr3+. Since
each mole of O2 can consume 4 mol of electrons to make H2O, then 1 mole of
Cr2O7 2- is equivalent to 1.5 mol of O2.

Number of milli moles of O2=450/32

Number of milli moles of Cr2O7 2- required=450/32*1.5=300/32=75/8 m.mol.

Molecular mass of Cr2O7 2-=216g/l

Weight of dichromate ion in 75/8 mmol=75*216/8 mg=2025mg/l

Therefore 2025mg/lit of dichromate ion is required to oxidise 450mg/l of cod.