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
