Problem: 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, SO2, acts as a diprotic acid in aqueous solution. At 25°C the acidity constants are:SO2 (aq) + 2H 2O (l) ⇋ HSO3– (aq) + H3O+ (aq)               K a1 = 1.54 x 10–2HSO3– (aq) + H2O (l) ⇋ SO32– (aq) + H3O+ (aq)              K a2 = 1.02 x 10–7Calculate the pH of a 0.1 M aqueous solution of sodium sulfite (Na  2SO3).A. 4.00B. 10.00C. 7.00D. 1.00E. 6.59

FREE Expert Solution

We’re being asked to calculate the pH of a 0.1 M aqueous solution of sodium sulfite (Na2SO3). Na2SO3 is an ionic compound it will break up into its ions in the solution:

Na2SO3(aq) 2 Na+(aq) + SO32-(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

• SO32- is the basic form of the diprotic acid H2SO3. A diprotic acid means it can donate two protons (H+and it will have two equilibrium reactions

H2SO3 Kb2Ka1 HSO3- Kb1Ka2 SO32-


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 Kb expression.
Step 3: Calculate for the equilibrium concentration.
Step 4: Calculate pOH.
Step 5: Calculate pH.

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Problem Details

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, SO2, acts as a diprotic acid in aqueous solution. At 25°C the acidity constants are:

SO2 (aq) + 2H 2O (l) ⇋ HSO3 (aq) + H3O(aq)               K a1 = 1.54 x 10–2
HSO3– (aq) + H2O (l) ⇋ SO32– (aq) + H3O(aq)              K a2 = 1.02 x 10–7

Calculate the pH of a 0.1 M aqueous solution of sodium sulfite (Na  2SO3).

A. 4.00

B. 10.00

C. 7.00

D. 1.00

E. 6.59

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