[sidharth]

which is more acidic among HCL -- H2O -- H2S ?
 
I would say HCL > H2O > H2S   looking at only electronegativity

but the actual answer  is hcl>h2s>h2o   --- 
1)  h2s>h2o    comparing the size of S and O (the -ve charge has more space to spread).
2)  But HCL is more acidic than both despite Cl having less electronegativity than oxygen

can we do this question based only on 1 particular logic?? :delta:

[Schrödinger]

I don't think using one particular logic can be of substantial help. You need to take into account both electronegativity and size.

Electronegativity is to be taken into account when you move across a period, because change in size is not as significant/pronounced as change in electronegativity.

Size is to be taken into account when you move down a group, because change in electronegativity is not as significant/pronounced as change in size.

So, using both reasons, you can arrive at the fact that H₂O<H₂S<HCl 
(You have move in a L-shaped manner in the periodic table from O to S to Cl )
Hope this helps