Two Guiding Principles for Predicting Relative Acidities

all acids are acting as proton donors to water, and the conjugate acid for all acids being compared is HsO. Therefore, in predicting the relative acidities we do not need to concern ourselves with the stability of HjO or H3O because it is the same for all the acids. 

we have two choices in predicting relative acidities we can focus on the relative stabilities of all the HA structures being compared, or we can focus on the relative stabilities of all the A s created by the various HAs. Since chemists are well versed in those factors that stabilize charges (induction, resonance, etc.), it is easiest for us to rationalize acidity differences by comparing charged species. Hence, two guiding principles emerge  

---