Given that it requires 27.9 ml of 0.270 m na2s2o3(aq) to titrate a 15.0-ml sample of i3–(aq), calculate the molarity of i3–(aq) in the solution.

The molar concentration of the KI_3 solution is 0.251 mol/L.

Step 1. Write the balanced chemical equation

I_3^(-) + 2S_2O_3^(2-) → 3I^(-) + S_4O_6^(2-)

Step 2. Calculate the moles of S_2O_3^(2-)

Moles of S_2O_3^(2-)

= 27.9 mL S_2O_3^(2-) ×[0.270 mmol S_2O_3^(2-)/(1 mL S_2O_3^(2-)]

= 7.533 mmol S_2O_3^(2-)

Step 3. Calculate the moles of I_3^(-)

Moles of I_3^(-) = 7.533 mmol S_2O_3^(2- × [1 mmol I_3^(-)/(2 mmol S_2O_3^(2-)] = 3.766 mmol I_3^(-)

Step 4. Calculate the molar concentration of the I_3^(-)

c = "moles"/"litres" = 3.766 mmol/15.0 mL = 0.251 mol/L

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