The Winkler method can be used to determine the amount of dissolved oxygen in water samples by converting dissolved oxygen to MnO₂ with MnSO₄ and reacting it with NaOH and KI. The solution is then acidified for the oxidation of I⁻ to I₃⁻ by MnO₂ to occur. The amount of I₃⁻ formed is determined by titration with S₂O₃²⁻. The unbalanced equations are

Mn²⁺(aq) + O₂(g)→ MnO₂(s)

MnO₂(s)+ I⁻(aq) →Mn2+(aq) + I₃⁻(aq)

I₃⁻(aq) + S₂O₃²⁻(aq) → I⁻(aq) + S₄O₆²⁻(aq)

When 0.1578 g of a water sample is analyzed, the reaction needs 17.55 mL of 0.1300 M Na₂S₂O₃. Calculate the mass percent of oxygen in the sample.