Question

What are the theoretical oxygen demand and organic carbon concentration of a water that contains the following components:

Glucose (C6H12O6): 150 mg/L

Benzene (C6H6):15 mg/L

Answer 1

Part a

Mass Concentration of glucose = 150 mg/L

Molar concentration = mass Concentration /molecular weight

= (150 mg/L) / (180.16 mg/mMol)

= 0.8326 mmol/L

The balanced reaction

C6H12O6 + 6O2 = 6CO2 + 6H2O

From the stoichiometry of the reaction

Theoretical oxygen demand for 1 mmol/L of glucose = 6 mmol/L

Theoretical oxygen demand for 0.8326 mmol/L of glucose

= 6 x 0.8326 = 4.996 mmol/L

Theoretical oxygen demand for 150 mg/L of glucose

= 4.996 mmol/L x 32 mg/mmol

= 159.86 mg/L

Part b

Mass Concentration of Benzene = 15 mg/L

Molar concentration = mass Concentration /molecular weight

= (15 mg/L) / (78.11 mg/mMol)

= 0.19204 mmol/L

The balanced reaction

C₆H₆ + 7.5 O₂ = 6 CO₂ + 3 H₂O

From the stoichiometry of the reaction

Theoretical oxygen demand for 1 mmol/L of Benzene = 7.5 mmol/L

Theoretical oxygen demand for 0.19204 mmol/L of Benzene

= 7.5 x 0.19204 = 1.4403 mmol/L

Theoretical oxygen demand for 15 mg/L of Benzene

= 1.4403 mmol/L x 32 mg/mmol

= 46.10 mg/L