Question

You have a solution of 5 mL of 0.100 M CH₃COOH.

a. What are its charge and mass balance equations?

b. Where do the hydronium ions mainly come from? The hydroxide ions?

c. What will its pH be at equilibrium?

Answer 1

Ans:

a) Charge and mass balance equations are

CH₃COOH CH₃COO^- + H⁺

H₂O + H⁺ H₃O⁺

b) Hydronium ions mainly comes from acetic acid molecules and there were no hydroxide ions produced.

c) CH₃COOH + H₂O CH₃COO^- + H₃0⁺

Initial concentration 0.1 M 0 0

At equilibrium concentration (0.1- x) x x

Ka = [CH₃COO^-][H₃0⁺ ]/[CH₃COOH] (Ka = dissociation constant = 1.8 X 10^-5 for acetic acid)

1.8 X 10^-5 = x.x/(0.1-x)

x ={ -1.8 X 10^-5 + [(1.8 X 10^-5 )² + 4(0.1)(1.8 X 10^-5 )]^1/2}/2 (x = concentration of H⁺)

= 0.001332 M

pH = -log[H⁺]

= -log(0.001332)

= 2.875