Question

Determine the theoretical oxygen demand for the following solutions:

(a) 200 mg/L of acetic acid, CH₃COOH

(b) 40 mg/L of ethanol. C₂H₅OH

(c) 64 mg/L of sucrose, C₂H₁₂O₆.

All reactions are of the form: CH₃COOH + O₂ >> CO₂ + H₂O

Answer 1

Determine the theoretical oxygen demand for the following solutions:

(a) 200 mg/L of acetic acid, CH₃COOH

(b) 40 mg/L of ethanol. C₂H₅OH

(c) 64 mg/L of sucrose, C₂H₁₂O₆.

All reactions are of the form: CH₃COOH + O₂ >> CO₂ + H₂O

theoretical oxygen demand means amount oxygen is required to oxidise the compound

a) CH₃COOH + O₂ >> CO₂ + H₂O

first balance the equation

CH₃COOH + 2O₂ >> 2CO₂ + 2H₂O

MW of acetic acid = 60    MW of oxygen = 64

Theoretical oxygen demand = (wt of acetic acid) * (MW of oxygen/ MW of oxygen)

Theoretical oxygen demand = (200 mg) * (64/60) = 213.33 mg /L

b) C₂H₅OH + 3O₂ >> 2CO₂ + 3H₂O

MW of ethanol = 46    MW of oxygen = 96

Theoretical oxygen demand = (wt of ethanol) * (MW of ethanol/ MW of oxygen)

Theoretical oxygen demand = (40 mg) * (46/96) = 19.16 mg /L

c) sucrose formula C₁₂H₂₂O₁₁

C₁₂H₂₂O₁₁ + 12O2 -à 12CO₂ + 11H₂O

MW of sucrose = 342    MW of oxygen = 384

Theoretical oxygen demand = (wt of sucrose) * (MW of sucrose/ MW of oxygen)

Theoretical oxygen demand = (64 mg) * (342/384) = 57 mg /L