[mistche20]

Everywhere it says that the greater the polarizing power of the metal, the greater acidity of the aquo complex. Soundsgood to me, if we are comparing metals in THE SAME PERIOD.

But what if we compare metals through the same group? according to this, 6 row metals would be less acidic, but on the other hand i'm thinking: metals from the 6th row have 5d orbitals, more extended, and their bonding with ligands (H2O) has some degree of covalency, hence greater acidity  due to stronger O-Metal bond.

the two are opposite!!!!

[gelminil]

I think you can figure this one out using charge density.
What is the charge of the metal ion and the size.
You can calculate charge density and metals of similar charge density with have similar acidity.  So for instance, Al3+ will have a large charge density and hence acidity versus Tl+.

[AWK]

http://www.google.pl/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CE8QFjAA&url=http%3A%2F%2Funcw.edu%2Fchem%2FCourses%2FHancock%2FCHM445%2FAcidity%2520of%2520metal%2520ions%2520in%2520aqueous%2520solution.doc&ei=4PS0T5WUAsep-gaX_a33DQ&usg=AFQjCNHHxo1HrKcSfzEADfvtwyRDeoonKA&sig2=pOSCKjVZzbX4Hs-6imDtLA

[cheese (MSW)]

http://www.chemguide.co.uk/inorganic/complexions/acidity.html