It has been shown that the undissociated form (i.e., the acid form) of acetic ac

Question

It has been shown that the undissociated form (i.e., the acid form) of acetic acid (HAc = CH3COOH) is toxic to the bacteria that make methane. The toxic level is reported to be 10 mg/L (= 1.67 x 10-4 M). The total acetic acid concentration (e.g., TOTAc) in an anaerobic digester is 6,000 mg/L as CH3COOH (= 0.10 M) and the pH is 6.5. Is the resulting level of HAc toxic to the bacteria? Show all calculations necessary to justify your answer. A yes or no without supporting calculations will receive no credit. The MW of CH3COOH is 60 g/mol.

Explanation / Answer

Given data:

Total acetic acid concentration = Dissociated AcOH + Undissociated AcOH = 6000 mg/L

pH of 0.1 m acetic acid is 6.5

It means initial concentration of AcOH = 0.1 M

Acetic acid ionizes as,

AcOH < -------------- > AcO– (aq.) + H+ (aq.)

pH = 6.5

pH = – log [H+]

log [H+]= –pH

[H+] = 10 –pH

[H+] = 10–6.5

[H+] = 3.2 x 10-7 M/L

At quilibrium, [AcOH] i.e. unionized AcOH concentration will be lowered by 3.2 x 10-7 M/L.

[AcOH] = 0.1 – [H+]

[AcOH] = 0.1 – (3.2 x 10-7)

[AcOH] = 0.0999 M/L

Molar of AcOH is 60 g

Hence

Mass of AcOH undissociated = 0.0999 x 60 = 5.999 g/L AcOH = 5999 mg/L AcOH ……..(as, 1 g = 1000 mg)

But toxic level is just 10 mg/L

Hence 0.10 M AcOH solution at pH = 6.5 is much above the toxic level for bacteria.

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