Question

The two pK_as of sulfurous acid (H₂SO₃) are 1.81 and 6.91, respectively. The predominant species at pH = 9 is: (Question from "Cracking the MCAT, 2010-2011 edition" by The Princeton Review)

		1. H+
		2. HSO3−
		3. SO32−
		4. H2SO3

Answer 1

Given pH = 9

We know that pH = - log [H⁺]

So [ H⁺] = 10 ^-pH

               = 10 ^-9

               = 1.0 x10 ^-9 M

It is not likely that a concentration of this magnitude is indicative of a predominate species at pH = 9

                                 ---> so we can omit choice (4)

At pH of 1.81, the titration of the first acidic proton is halfway complete, and at a pH of 9, there is not likely to be a measureable quantity of H₂SO₃ left. Therefore we can omit choice (1)

At pH of 6.91, the titration of the second acidic proton is half way complete and at this point [HSO₃^-]=[SO₃^2-] Therefore, for any pH greater than 6.91 , [HSO₃^-]<[SO₃^2-] so we can omit choice (2)

Therefore the correct answer is choice (3)