Chemistry Practice Problems Diprotic Acid Practice Problems Solution: In the atmosphere, sulfur dioxide and nitrogen dio...

Solution: 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 concentration of sulfite (SO32–) in the above solution.

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 10–2
HSO3– (aq) + H2O (l) ⇋ SO32– (aq) + H3O(aq)              K a2 = 1.02 x 10–7

Calculate the concentration of sulfite (SO32–) in the above solution.

Solution

We are being asked to calculate the concentration of sulfite (SO32) in the solution if the concentration of sulfur dioxide in water saturated with sulfur dioxide is 1.5 M


We're going to calculate the concentration of sulfite (SO32–) in two parts:

1st Part: Calculate the concentration of hydrogen sulfite (HSO3) using the 1st equilibrium reaction:

Step 1: Construct an ICE chart for the 1st equilibrium reaction to calculate for the concentration of
Step 2: Write the Ka1 expression.
Step 3: Calculate for the equilibrium concentration.

2nd Part: Calculate the concentration of sulfite (SO3) using the 2nd equilibrium reaction:

Step 1: Construct an ICE chart for the 1st equilibrium reaction to calculate for the concentration of
Step 2: Write the Ka2 expression.
Step 3: Calculate for the equilibrium concentration.


1st Part: Calculate the concentration of hydrogen sulfite (HSO3) using the 1st equilibrium reaction:

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

Since we’re dealing with a weak acid and Ka is an equilibrium expression, we will have to create an ICE chart to determine the equilibrium concentration of each species:

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