Question

Calculate the molarity of S₂O₃²⁻ if 3.0 mL 0.10 M K₂S₂O₈, 2.0 mL 0.10 M K₂SO₄, 1.0 mL 2% starch, 2.0 mL 0.010 M Na₂S₂O₃, and 5.0 mL 0.20 M KI are mixed. (Include units in your answer)

Answer 1

The S₂O₃^2- ONLY can come from the Na₂S₂O₃ since nothing else has it (in other problems, the material you are asked to find the concentration may come from more than 1 of the compounds).

Equation 1: 2 I - (aq) + S₂O₈^2- (aq) --> I₂ (aq) + 2 SO₄^2- (aq)

Equations 2: (very fast) I₂(aq) + 2 S₂O₃^2- (aq) --> 2I ^- (aq) + S₄O₆^2- (aq)

1 mL = 0.001 L
2 mL = 0.002 L
0.010 M Na₂S₂O₃ = 0.010 mole/L Na₂S₂O₃

0.010 mole/L * 0.002 L = 2*10^-5 mole Na₂S₂O₃

2*10^-5 mole of Na₂S₂O₈ contains mole 2*10^-5 mole of S₂O₃²⁻ from the chemical formula.

Total volume =  3 mL + 2 mL + 1 mL + 2 mL + 5 mL = 13 mL = 0.013 L

Concentration of S₂O₃²⁻ = 2*10^-5mole / 0.013 L = 0.0015 M