Chapter 4, Problem 4.156MP

Brass is an approximately 4:1 alloy of copper and zinc, along with small amounts of tin, lead, and iron. The mass percents of copper and zinc can be determined by a procedure that begins with dissolving the brass in hot nitric acid. The resulting solution of ${\text{Cu}}^{2+}$ and ${\text{Zn}}^{2+}$ ions is then treated with aqueous ammonia to lower its acidity, followed by addition of sodium thiocyanate (NaSCN) and sulfurous acid $\left({\text{H}}_{\text{2}}{\text{SO}}_{\text{3}}\right)$ to precipitate copper(I) thiocyanate (CuSCN). The solid CuSCN is collected, dissolved in aqueous acid, and treated with potassium iodate $\left({\text{KIO}}_{\text{3}}\right)$ to give iodine, which is then titrated vith aqueous sodium thiosulfate $\left({\text{Na}}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_{\text{3}}\right)$ . The filtrate remaining after CuSCN has been removed is neutralized by addition of aqueous ammonia, and a solution of diammonium hydrogen phosphate $\left({\left({\text{NH}}_{\text{4}}\right)}_{\text{2}}{\text{HPO}}_{\text{4}}\right)$ is added to yield a precipitate of zinc ammonium phosphate $\left({\text{ZnNH}}_{\text{4}}{\text{PO}}_{\text{4}}\right)$ . Heating the precipitate to 900 °C converts it to zinc pyrophosphate $\left({\text{Zn}}_{\text{2}}{\text{P}}_{\text{2}}{\text{O}}_{\text{7}}\right)$ , which is weighed. The equations are
(1) $\text{Cu}\left(s\right)+{\text{NO}}_3{}^{-}\left(aq\right)\to {\text{Cu}}^{2+}\left(aq\right)+\text{NO}\left(g\right)\left(\text{in acid}\right)$
(2) ${\text{Cu}}^{2+}\left(aq\right)+{\text{SCN}}^{-}\left(aq\right)+{\text{HSO}}_3{}^{-}\left(aq\right)\to \text{CuSCN}\left(s\right)+{\text{HSO}}_{\text{4}}{}^{-}\left(aq\right)\left(\text{in acid}\right)$
(3) ${\text{Cu}}^{+}\left(aq\right)+{\text{IO}}_3{}^{-}\left(aq\right)\to {\text{Cu}}^{2+}\left(aq\right)+{\text{I}}_2\left(aq\right)\left(\text{in acid}\right)$
(4) ${\text{I}}_{\text{2}}\left(aq\right)+{\text{S}}_{\text{2}}{\text{O}}_3{}^{2-}\left(aq\right)\to {\text{I}}^{-}\left(aq\right)+{\text{S}}_4{\text{O}}_6{}^{2-}\left(aq\right)\left(\text{in acid}\right)$
(5) ${\text{ZnH}}_{\text{4}}{\text{PO}}_4\left(s\right)\to {\text{Zn}}_{\text{2}}{\text{P}}_{\text{2}}{\text{O}}_{\text{7}}\left(s\right)+{\text{H}}_{\text{2}}\text{O}\left(g\right)+{\text{NH}}_3\left(g\right)$
(a) Balance all equations.
(b) When a brass sample with a mass of 0.544 g was subjected to the preceding analysis, 10.82 mL of 0.1220 M sodium thiosulfate was required for the reaction with iodine. What is the mass percent copper in the brass?
(c) The brass sample in part (b) yielded 0.246 g of ${\text{Zn}}_{\text{2}}{\text{P}}_{\text{2}}{\text{O}}_{\text{7}}$ . What is the mass percent zinc in the brass?