[kenny1999]

I would like to know the chemistry, especially the acid behaviour of sulphuric acid

what will it be when it alone dissolves in water? It will form mainly H+ with Hydrogensulphate or it will form H+ with sulphate ?

I encountered a M.C. question that requires me to find out "the pH of a 0.0020M H2SO4". At first, I suppose for 1 mole of H2SO4 it will gives 2 mole of H+ since it is dibasic so I multiply the concentration by 2 to give [H+], however, the answer turns out to be that the concentration of H+ [H+] = 0.0020 instead of 0.0040 so I think that
they suppose the sulphuric acid only gives 1 ion.

So, I wish to know how to determine if a sample of sulphuric acid that will give 1 or 2 H+ ion (for 1 molecule of H2SO4 disssolved)

Thanks!

[DevaDevil]

find the pKa values of sulfuric acid.
then you can calculate from the equilibria if the second proton stays or goes.

(as a rule of thumb: under most circumstances the second proton dissociates as well)

[kenny1999]

find the pKa values of sulfuric acid.
then you can calculate from the equilibria if the second proton stays or goes.

(as a rule of thumb: under most circumstances the second proton dissociates as well)

sorry I just have little bit concept of equilibria - that a reaction would occur in both forward and backward direction and reach a steady state of amount of reactants and products, but I don't know about the detailed calculation.

As you said if under most circumstances the second proton would dissociate so how
can I know that in that M.C. questions that it dissociates 1 H+ atom?

(and the fact that the solution book explained that 0.0020M of sulphuric acid gives 1 H+ atom so the pH is 2.69, but it just doesn't explain why under such a circumstance I need to assume 1 H+ atom!! ) I am very frustrated and confused.

[DevaDevil]

I do not know why they assume only 1 proton dissociates. In any case if they DO asume that, they are wrong.

[Borek]

This is tricky. Second proton is dissociated in about 75%.

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-simplified

[khimani]

By my opiniun,
H2SO4 is a di basic acid so it dissociated 2 H+ ion.
thus, 0.002 M H2SO4 dissociated 2 H+ ion.
so you consider the 0.004 M concentration to calculate pH.
pH= -log[0.004]
    = 2.69

[Borek]

By my opiniun,
H2SO4 is a di basic acid so it dissociated 2 H+ ion.
thus, 0.002 M H2SO4 dissociated 2 H+ ion.
so you consider the 0.004 M concentration to calculate pH.
pH= -log[0.004]
    = 2.69

That's only an approximation, real pH is slightly different. pKa2 = 2.0, so it is not really strong.