[subrata]

How to take pH into consideration when doing stoichiometric calculations? Like the amount of acid needed to turn a solution into 1.5, 2.0 pH will be different. So how to do stiochiometric calculations taking pH into consideration? I am giving an equation

Na2SO3 + H2SO4 = Na2SO4 + SO2 + H2O

Here if i want to make pH of solution 1.5 or 2 the amount of H2SO4 reqd will be different. How do calculate it?

[Borek]

Assuming it goes according to the reaction you wrote, pH will be controlled by the excess acid.

[subrata]

Yes thats ok. But how do i do the stoichiometric calculation?

[Borek]

Calculate what final concentration of acid you need to get a given pH, calculate how much you need to react with whatever base is present in the solution - and add.

[subrata]

Thats not the answer im looking for. Let me make myself more clear. Suppose the reaction is taking place in a reactor, now by stoichiometry 123 gms of na2so3 will react with 98 gms of h2so4. Now tell me the amount of acid if want to keep the pH of the solution 1.5 and 2 respectively.

[Borek]

If that's not the answer you are looking for, I don't know what you are looking for - as that's the answer that fits the question you ask. Perhaps you should take into account fact that sulfuric acid Ka2 is relatively small, but it will still count as pH of excess sulfuric acid.