Which acid of each pair shown here would you expect to be stronger?

CH₃CO₂H or CH₂FCO₂H

The +I effect of –CH₃ group increases the electron density on the O-H bond. Therefore, the release of proton becomes difficult. On the other hand, the -I effect of F decreases the electron density on the O-H bond. Therefore, the proton can be released easily. Hence, CH₂FCO₂H is a stronger acid than CH₃CO₂H.

NOTE:A strong acid is an acid which dissociates completely releasing almost all of its protons in the solution.