A chemist investigates the reaction between molecular iodine and thiosulfate ion in…

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

A chemist investigates the reaction between molecular iodine and thiosulfate ion in aqueous solution. She dissolves

0.0635\,\text{g}

I_2

(molar mass

253.8\,\text{g mol}^{-1}

) in water and titrates the solution with

50.2\,\text{mL}

0.0250\,\text{M}

Na_2S_2O_3

, reaching a sharp endpoint with starch indicator. Use the experimental data to analyze the iodine–thiosulfate reaction. (a) Calculate the number of moles of

I_2

that were present at the start of the titration. (b) Calculate the number of moles of

S_2O_3^{2-}

delivered from the buret. (c) Using your answers from parts (a) and (b), deduce the simplest whole-number stoichiometric coefficients for

I_2

and

S_2O_3^{2-}

and write the balanced net-ionic equation for the reaction that occurs. (d) If

0.500\,\text{mmol}

I_2

reacts according to your balanced equation from part (c), determine the mass of

I^-

ions produced, and explain how your calculation confirms conservation of iodine atoms.

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