Question

3) The company you work for plans to release a waste stream containing 10 mg/L of phenol (C6H5OH). Calculate the theoretical oxygen demand of this waste stream. It may be helpful to use the following (unbalanced) chemical equation and to remember that ThOD should be reported in mg 02/L. CoH5OH (s) + __ 02 (g) → __CO2 (g) + H20 (1)

Answer 1

Q3. Balanced reaction : C6H5OH(s) + 7 O2(g) $\rightarrow$ 6 CO2(g) + 3 H2O(l)

mass concentration of C6H5OH = 10 mg/L

Let volume of solution = 1 L

mass of phenol = (mass concentration of C6H5OH) * (volume of solution)

mass of phenol = (10 mg/L) * (1 L)

mass of phenol = 10 mg

moles of phenol = (mass of phenol) / (molar mass C6H5OH)

moles of phenol = (10 mg) / (94.1 g/mol)

moles of phenol = 0.106 mmol

moles of O2 required = (moles of phenol) * (7 moles O2 / 1 mole C6H5OH)

moles of O2 required = (0.106 mmol) * (7 / 1)

moles of O2 required = 0.744 mmol

mass of O2 required = (moles of O2 required) * (molar mass of O2)

mass of O2 required = (0.744 mmol) * (32.0 g/mol)

mass of O2 required = 23.8 mg

Theoretical oxygen demand = (mass of O2 required) / (volume of solution)

Theoretical oxygen demand = (23.8 mg) / (1 L)

Theoretical oxygen demand = 23.8 mg/L