Chapter 5, Problem 92RQ

Methylbromide, ${\text{CH}}_{\text{3}}\text{Br}$, is used in agriculture to fumigate soil to rid it of pests such as nematodes. It is injected directly into the soil, but over time it has a tendency to escape before it can undergo natural degradation to innocuous products. Soil chemists have found that ammonium thiosulfate, ${\left({\text{NH}}_{\text{4}}\right)}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_{\text{3}}$, a nitrogen and sulfur fertilizer, drastically reduces methylbromide emissions by causing it to degrade. In a chemical analysis to determine the purity of a batch of commercial ammonium thiosulfate, a chemist first prepared a standard solution of iodine following the procedure in Problem 5.91. First, 0.462 g ${\text{KlO}}_{\text{3}}$ was dissolved in water to make 125 mL of solution. The solution was made acidic and treated with excess potassium iodide, which caused the following reaction to take place:

${\text{IO}}_{\text{3}}{}^{\text{-}}{\text{+8I}}^{\text{-}}{\text{+6H}}^{\text{+}}\to {\text{3I}}_{\text{3}}{}^{\text{-}}{\text{+3H}}_{\text{2}}\text{O}$

The solution containing the ${\text{I}}_{\text{3}}{}^{\text{-}}$ ion was then diluted to exactly 250.0 mL in a volumetric flask. Next, the chemist dissolved 0.2180 g of the fertilizer in water, added starch indicator, and titrated it with the standard ${\text{I}}_{\text{3}}{}^{\text{-}}$ solution. The reaction was

$2{\text{S}}_2{\text{O}}_3{}^{2-}+{\text{I}}^{\text{-}}{}_3\to {\text{S}}_4{\text{O}}_6{}^{2-}+3{\text{I}}^{\text{-}}$

The titration required 27.99 mL of the ${\text{I}}_{\text{3}}{}^{\text{-}}$ solution.

(a) What was the molarity of the ${\text{I}}_{\text{3}}{}^{\text{-}}$ solution used in the titration?

(b) How many grams of ${\left({\text{NH}}_{\text{4}}\right)}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_{\text{3}}$ were in the fertilizer sample?

(c) What was the percentage by mass of ${\left({\text{NH}}_{\text{4}}\right)}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_{\text{3}}$ in the fertilizer?