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Back titration of NaOH with HCl and reason for a back titration

A 40 0 L of N2 gas containing SO2 gas as an impurity was bubbled through a 3% solution of H2O2  The SO2 was converted to H2SO4:


A 25.0 ml portion of 0.0100 mol/l NaOH was added to the solution, and the excess was back-titrated with 13.6 ml of 0.0100 mol/l HCl. Calculate the parts per million of S029that is, ml S02/10 6 ml sample) if the sensity of SO2 is 2.85 g/l ?

I was wondering why do they have to a back-titration?
is it not easier to do a direct titration between H2SO4 and NaOH?


Sulfuric acid is a diprotic acid, when titrating a diprotic acid you end up with two transition points, or two endpoints.  Titrating a diprotic acid is more tricky than a monoprotic acid, since you may need to use two indicators that do not conflict with each other.

By reacting with excess sodium hydroxide you make sure that all of the acid has been neutralized and you only need to worry about 1 end point.