In the below question, if I find no of moles of H+ from individual solution and add up and then divide by three times the volume I get the conc of H+ in soln.Then negative log of that gives me one value(not in options).But if I do the same using OH- (finding conc of OH- in final soln and then finding H+(10^-14/[OH-]) I get first option .Could someone clarify this why am I getting different answers
Whenever in the ques, ph is given and if it's less than 7, do the calculations by taking conc of [H+] and it ph is more than 7 go for [OH-] concentration initially.
If [OH]- ion concentration is greater than [H]+ ion concentration, then you will find different answer if you solve through [H]+ ion conc. procedure. Since, [OH]- ion is dominating here, take [OH]- ion procedure into consideration.
When you face a problem in which [H]+ ion conc. > [OH]- ion concentration, take [H]+ ion approach.
When [OH]- ion > [H]+ ion, take [OH]- ion approach