ARIO is a convenient mnemonic to help you analyze and compare compound acidities and pKas
If you are struggling with what to look for when faced with a problem about comparing compound acidities, use the ARIO system. This stands for Aromaticity, Resonance, Induction and Orbitals in that order, meaning that you would first check the conjugate bases (ALWAYS deprotonate the acids first to get the conjugate bases) for aromaticity (cyclic pi electron system). If one is and the other isn't, then the problem is most likely based around that concept. If not, move on down the line to checking for possibility of resonance structures and so on.
The ARIO concept is widely taught in introductory organic courses, and so further reading about it is always at arms length.
This handy handout summarizes the ARIO concept with several examples.
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