Question

pH of Aqueous Sulfuric Acid Calculate the pH of a 2.07×10-3 M solution of H2SO4. (Ka = 0.0120 for HSO4-)

Answer 1

Note that first dissociation of H₂SO₄ is stoichiometric since H₂SO₄ is a strong acid.

The first dissociation can be written as

Given that the starting concentration of H₂SO₄ is .

Note that H₂SO₄ will completely dissociate into H⁺ and HSO₄^- as predicted by the above reaction.

Hence, the concentrations of species after first dissociation can be written as

Now, HSO₄^- is a weak acid and will only partially dissociate to form some H⁺.

It can be written as

Now, we can create an ICE table to calculate the equilibrium H⁺ concentration after the second dissociation.

Initial, M			0
Change, M	-x	+x	+x
Equilibrium, M			x

Now, we can write the expression of K_a as follows using the ICE table

hence, the equilibrium H⁺ concentration can be calculated as follows:

Now, pH is defined as the negative logarithm of H⁺ ion concentration.

Hence,

Hence, the pH of the solution is approximately 2.44.