[Wil"]

1. Why H3O+ can be written as H+??
I once have asked my teaher, but apparantly I didn't really get her point.

2. what does this sentence mean?
"Since the acidic properties of the proton will not be affected by the degree of hydration, we can write H3O+ as H+".
if more water presents, will the concentration of the acidity be decreased??

Please answer them~~~

[AWK]

There are a few acids theories. Writing down H3O+ or H+ differentiates between Bronsted theory and Arrhenius theory. This is only notation. In both theories acidity constants are the same. Writing down H3O+ is only a conventional. In fact H+ in solution contacts with more than one molecule of water, but we should in mind possibility of easy balancing reactions in integers.
Such ions as H3O+, H5O2+ and H7O3+ are found in hydrates  of HClO4 and superacids in their crystal structures.

[shelanachium]

All cations are to some extent hydrated in aqueous solution, but most can be identified as 'anhydrous' cations in crystal structures and have fairly well-defined radii e.g. Na+ seems to be much the same size in all sodium salt crystal structures.

H+ however is a naked proton, 100,000 times smaller than any intact atom or cation derived from other elements, and never exists as a 'free' proton, even in the strongest superacid crystal structures. So some prefer to write H3O+ for the species present in aqueous acids because H3O+ is an identifiable component of definite size in many crystal structures such as the 'hydrates' of perchloric acid and superacids mentioned by AWK.