Question

Acrylic acid, whose formula is HC3H3O2 or HO2CCH double bonded to CH2 , is used in the manufacture of plastics. A 0.45 M aqueous solution of acrylic acid has a pH of 2.30.

What is Ka for acrylic acid?

Answer 1

Acrylic acid has formula CH₂=CH-COOH. Its dissociation can be shown as

CH₂=CH-COOH + H₂OCH₂=CH-COO^- + H3O⁺

Initial concentration (M) 0.45 0 0

Change (M) -x +x +x

Equilibrium concentration (M) 0.45-x x x

Acid dissociation constant for CH₂=CH-COOH

Ka=[CH₂=CH-COO^-][H₃O⁺]/[CH₂=CH-COOH]

Ka=x²/0.45-x ............equation (1)

where x=[CH₂=CH-COO^-]=[H₃O⁺] (equilibrium concentrations)

Also given pH of acrylic acid=2.30

-log[H₃O⁺]=2.30

[H₃O⁺]0.005 M=x

So substituting x=0.005 M in equation (1)

Ka=(0.005)²/0.450-0.005

=0.000025/0.445

=0.000056179

5.62x10^-5

So Ka for acrylic acid=5.62x10^-5