Which acid is the weakest in aqueous solution?

• A. acetic acid (Ka = 1.8 × 10^-5)
• B. formic acid (Ka = 1.8 × 10^-4)
• C. hydrocyanic acid (Ka = 6.2 × 10^-10)
• D. nitrous acid (Ka = 4.5 × 10^-4)

Answer: (C)

Strength in water is governed by the acid dissociation constant, Ka. A larger Ka means more dissociation into H+ and A−, so a stronger acid; a very small Ka means little dissociation and a weaker acid.

Hydrocyanic acid has Ka = 6.2 × 10^-10, which is far smaller than the Ka values of the other acids listed. That tiny Ka indicates it barely donates protons to water, making it the weakest acid among them. The others—with Ka on the order of 10^-5 to 10^-4—dissociate much more and are correspondingly stronger acids.