Question

1. [H2SeO3]is a diprotic acid with pK_a1= 2.65  and pK_a2=  7.9
   1. Write the balanced chemical equations that allow definition of the K_avalues of [H2SeO3].

                      +                                  D

                      +                                  D

2. Sketch and label the distribution diagram for [H2SeO3],showing pH from -1 to +14, by filling in tables 1 and 2.

Table 1: 1^stacidic proton

a
pH
a

Table 2: 2^ndacidic proton

a
pH
a

c. State the chemical formula for the major species present at the following pH values_:

pH	0.1	1.3	2.2	3.5	4.3	5.5	7.0
Major Species
pH	8.1	9.5	10.8	11.1	12.9	13.5	14.0
Major Species

Answer 1

The equilibriums are:

• For Ka1: H₂SO₃ ⇋ H⁺ + HSO₃^-
• For Ka2:   HSO₃^- ⇋ H⁺ + SO₃^2-

The species distribution diagram is:

Where S₂O₅^2- and HS₂O₅^- are negligible (you can ignore them). Also, for approximation, you can consider SO₂ as equal to H₂SO₃ (this species does not actually exist in aqueous solution, but we will asume it does).

I'm not quite sure what the table is supposed to have. If they are percentages of each species at given pH values, you can draw those values directly from the graph.

Regarding major species, we take into account that the pKa is the pH value at which there is a 50:50 relation between the involved species in that equilibrioum. Below pKa, the most acid species will predominate, and above pKa, the most basic species will predominate. Considering the informed pKas:

pH	0.1	1.3	2.2	3.5	4.3	5.5	7.0
Major species	H2SO3	H2SO3	H2SO3	HSO3-	HSO3-	HSO3-	HSO3-
pH	8.1	9.5	10.8	11.1	12.9	13.5	14.0
Major species	SO32-	SO32-	SO32-	SO32-	SO32-	SO32-	SO32-