The following process has been used to obtain iodine from oil-field brines in California.

$\begin{array}{l}\mathrm{NaI}+{\mathrm{AgNO}}_3\stackrel{ }{\to }\mathrm{AgI}+{\mathrm{NaNO}}_3.....\left(1\right)\\ 2\mathrm{AgI}+\mathrm{Fe}\stackrel{ }{\to }{\mathrm{FeI}}_2+2\mathrm{Ag}......\left(2\right)\\ 2{\mathrm{FeI}}_2+3{\mathrm{Cl}}_2\stackrel{ }{\to }2{\mathrm{FeCl}}_3+2{\mathrm{I}}_2.......\left(3\right)\end{array}$

How many grams of ${\mathrm{AgNO}}_3$ are required in the first step for every 254 kg ${\mathrm{I}}_2$ produced in the third step ?

 Balanced equation :

 Balance the given series of equations to get 2 moles of  Iodine

$\begin{array}{l}\mathrm{NaI}+{\mathrm{AgNO}}_3\stackrel{ }{\to }\mathrm{AgI}+{\mathrm{NaNO}}_3.....\left(1\right)\times 4\Rightarrow 4\mathrm{NaI}+4{\mathrm{AgNO}}_3\to 4\mathrm{AgI}+4{\mathrm{NaNO}}_3\\ 2\mathrm{AgI}+\mathrm{Fe}\stackrel{ }{\to }{\mathrm{FeI}}_2+2\mathrm{Ag}......\left(2\right)\times 2\Rightarrow 4\mathrm{AgI}+\text{\hspace{0.17em}2Fe\hspace{0.17em}}\stackrel{ }{\to }\text{\hspace{0.17em}\hspace{0.17em}2F}{\mathrm{eI}}_2+\text{\hspace{0.17em}\hspace{0.17em}4}\mathrm{Ag}\\ 2{\mathrm{FeI}}_2+3{\mathrm{Cl}}_2\stackrel{ }{\to }2{\mathrm{FeCl}}_3+2{\mathrm{I}}_2.......\left(3\right)\end{array}$

$$\begin{gathered} \mathrm{Hence} 4 \mathrm{moles} \mathrm{of} {\mathrm{AgNO}}_3\to 2 \mathrm{moles} \mathrm{of} {\mathrm{I}}_2 \\ 4\times 170\mathrm{g} \mathrm{of} {\mathrm{AgNO}}_3 \to 2\times 254\mathrm{g} \\ ? \to 254\times {10}^3\mathrm{g} \end{gathered}$$

$\mathrm{mass} \mathrm{of} {\mathrm{AgNO}}_3 \mathrm{required} = \frac{254\times {10}^3}{2\times 254}\times 4\times 170 = 340\times {10}^3\mathrm{g}$