Question

Rank the acidity of the following molecules: CH₃COOH (acetic acid), CF₃COOH (trifluoroacetic acid), H₂CO₃ (carbonic acid), H₂SiO3 (silicic acid).

Answer 1

A larger equilibrium constant means more H₃O⁺ and A^–, less HA. We can say that

the acid has a stronger tendency to donate its H⁺, which is the definition of a stronger acid.

Ka for acetic acid 1.75 × 10 ⁻⁵

Ka for CF₃COOH (trifluoroacetic acid)    1.00

Ka for H₂CO₃ (carbonic acid), 4.3 x 10-7

Ka for H2SiO3 (silicic acid) <1.00

The order of increasing acidity is following:

H₂CO₃ (carbonic acid), 4.3 x 10-7< H₂CO₃ (carbonic acid), 4.3 x 10-7< CF₃COOH (trifluoroacetic acid)    1.00< H2SiO3 (silicic acid) <1.00