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 hydrogen sulfite (NaHSO 3).A. 8.80B. 1.81C. 7.00D. 4.40E. cannot be determined

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We are being asked to calculate the pH of a 0.1 M aqueous solution of sodium hydrogen sulfite (NaHS3). NaHSO3 is an ionic compound and it will break up into its ions in the solution:

NaHSO3(aq) Na+(aq) + HSO3-(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

• HSO3- is an amphiprotic species meaning it acts both as proton donor and acceptor

H2SO3 Kb2Ka1 HSO3- Kb1Ka2 SO32-


When an amphiprotic salt is added to water, the pH of the solution can be estimated using the following equation:

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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 hydrogen sulfite (NaHSO 3).

A. 8.80

B. 1.81

C. 7.00

D. 4.40

E. cannot be determined

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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.