My book says
To compare the acidities of two compounds, compare the stability of the anions formed by loss of a proton with respect to the original compound.
Suppose we have acetic acid and ethyl alcohol. Now on losing a proton, they form the respective anions:
My book again says
The acetate anion is resonance stabilized whereas in the ethoxide ion the charge is localized on the oxygen atom. Hence, acetate ion is more stable as compared to the ethoxide ion and hence, acetic acid is the stronger acid.
The problem is we have only compared the stabilities of the anions with respect to each other and not with respect to the original compounds. So how do we know that it is correct?