I didnt really understand your question but from what i understand:if it involves onle one Fe3+ then y do we mutiply the conc wid 2....SO42- = weak base ...it doesnt affect the pH...isnt tht so?
We multiply it by 2 as the conc of Fe2(SO4)3 is 0.1. The equation is given for just 1 mole, [Fe(H20)6]^+2 is the same as Fe^+3 (aq).
The acid dissociation constant will stay the same as a change in conc. Of the acid will also change the conc of the products. Thus we need not worry about Ka and just use the conc. Of the acid as 0.2
Generally speaking, t does affect the pH but only to a very small extent, that WE can term it negligible, For stronger acid, the change is greater which is why we say there is a change.
Again i didnt quite understand what was your question, so i just typed in what I assumed you were asking