Sulfur dioxide gas is commonly used as a preservative in wine. An important source of SO₂ is solid sodium
metabisulfite (Na₂S₂O₅; molar mass 190 g/mol–). Na₂S₂O₅ reacts readily with acid as follows.
Na₂S₂O₅(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + 2SO₂(g)

Calculate the volume, in litres, of SO₂ produced at 1.00 atm pressure and 15.0^oC when 250 g of Na₂S₂O₅ reacts with excess acid.
Solution

The concentration of an aqueous solution of SO₂ (solution A) is to be determined using its reaction with
an aqueous solution of triiodide ions (I3). The relevant half reactions are
SO₂(aq) + 2H₂O(l) → 4H⁺(aq) + SO₄^-2–(aq) + 2e–

I₃ ^-(aq) + 2e– → 3I–^-(aq)

50.0 mL of a 0.0125 M solution of I₃ ^- is added to 50.0 mL of solution A, providing excess of I₃ ^-.

–The final 100.0 mL solution is called solution B.
Write an overall balanced chemical equation for the reaction that occurs, identifying the substance that is the reductant.
Solution

Calculate the amount, in mol, of I₃ ^- added to solution A.

Solution

The excess I₃ ^- remaining in the solution is determined by titration with a standard solution of sodium
thiosulfate (Na₂S₂O₃). The equation for the reaction is
S₂O₃^-2–(aq) + I₃ ^- (aq) + H₂O(l) → 3I–^-(aq) + S₂O₄^-2–(aq) + 2H⁺(aq)

14.70 mL of a 0.00850 M solution of Na₂S₂O₃ reacts exactly with all the I₃ ^- remaining in solution B.
Calculate the original concentration of SO₂ in solution A.
Solution