We’re being asked to calculate the pH of a 0.1 M aqueous solution of sodium sulfite (Na_{2}SO_{3}). Na_{2}SO_{3} is an ionic compound it will break up into its ions in the solution:

**Na _{2}SO_{3(aq)}**

• Main group metals should have a +3 charge or higher to be considered acidic. Na^{+} has only a +1 charge and is a neutral ion so it will not contribute to the pH of the solution

• SO_{3}^{2-} is the **basic **form of the diprotic acid H_{2}SO_{3}. A diprotic acid means it can donate two protons (H^{+}) and it will have two equilibrium reactions.

${\mathbf{H}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{3}}\mathbf{}\underset{{\mathbf{K}}_{{\mathbf{b}}_{\mathbf{2}}}}{\overset{{\mathbf{K}}_{{\mathbf{a}}_{\mathbf{1}}}}{\mathbf{\rightleftharpoons}}}\mathbf{}{{{\mathbf{HSO}}}_{{\mathbf{3}}}}^{{\mathbf{-}}}{\mathbf{}}\underset{{\mathbf{K}}_{{\mathbf{b}}_{\mathbf{1}}}}{\overset{{{\mathbf{K}}_{\mathbf{a}}}_{\mathbf{2}}}{\mathbf{\rightleftharpoons}}}{\mathbf{}}{{{\mathbf{SO}}}_{{\mathbf{3}}}}^{\mathbf{2}\mathbf{-}}$

**We're going to use the following steps to calculate for the pH of the solution: **

Step 1: Construct an ICE chart for the equilibrium reaction.

Step 2: Write the K_{b} expression.

Step 3: Calculate for the equilibrium concentration.

Step 4: Calculate pOH.

Step 5: Calculate pH.

In the atmosphere, sulfur dioxide and nitrogen dioxide react with water to give sulfurous acid and nitric acid. This produces “acid rain” which has a pH below 7 and can dissolve limestone.

Sulfur dioxide, SO_{2}, acts as a diprotic acid in aqueous solution. At 25°C the acidity constants are:

SO_{2} (aq) + 2H _{2}O (l) ⇋ HSO_{3}^{–} (aq) + H_{3}O^{+ }(aq) K _{a1} = 1.54 x 10^{–2}

HSO_{3}^{– }(aq) + H_{2}O (l) ⇋ SO_{3}^{2– }(aq) + H_{3}O^{+ }(aq) K _{a2} = 1.02 x 10^{–7}

Calculate the pH of a 0.1 M aqueous solution of sodium sulfite (Na _{2}SO_{3}).

A. 4.00

B. 10.00

C. 7.00

D. 1.00

E. 6.59

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Diprotic Acid concept. You can view video lessons to learn Diprotic Acid. Or if you need more Diprotic Acid practice, you can also practice Diprotic Acid practice problems.