Question

The self protonation constant for the solvent ethanol, CH₃CH₂OH (l) at 25 C is 8.0x10^-20. The self-protonation equilibrium is defined as:

CH₃CH₂OH (l) + CH₃CH₂OH (l) === CH₃CH₂OH₂⁺ (alc) + CH₃CH₂O^- (alc)

Where alc is the alcohol solution

(a)If we define the pH scale in alcohol by the equation: pH = -log [CH₃CH₂OH₂⁺], what is pH of a neutral alcohol solution?

(b)What is the pH of a 0.001 M solution of sodium ethoxide (CH₃CH₂ONa), in alcohol? Assume complete dissociation.

(c)What is the pH of 0.05 M acetic acid in alcohol?

(d)Is acetic acid (CH₃COOH) a stronger or a weaker acid in alcohol than in water? Explain.

Data: pK_a of acetic acid is 7.0 in alcohol and 4.8 in water

Answer 1

For self protonation reaction of ethanol,

(a) pH of neutral alcohol

pH = -log[C2H5OH]

(b) pH of 0.001 M C2H5ONa

Salt hydrolyzes

C2H5O- + C2H5OH ---> C2H5OH + C2H5O-

pH = 7

(c) pH of 0.05 M acetic acid in alcohol

CH3COOH <==> CH3COO- + H+

1.8 x 10^-5 = x^2/0.05

x = [H+] = 9.50 x 10^-4 M

pH = -log[H+] = 3.02

(d) acetic acid is weaker acid in alcohol than in water.

pKa of acetic acid is higher in alcohol than that in water, that is lower Ka in alcohol than in water. Ka is directly proportional to the dissociation extent for the acid. Therefore, it is a stronger acid in water than in alcohol.