Question

$25mL$ of asample of saturated solution of $Pb{I}_2$, requires $10mL$ of a certain $AgN{O}_3$ (aq) for its titration. What is the molarity of this $AgN{O}_3$(aq)? $K_{sp}$ of $Pb{I}_2=4\times {10}^{-9}$.

• A. $5\times {10}^{-3}M$
• B. $5\times {10}^{-1}M$
• C. $5\times {10}^{-5}M$
• D. $5\times {10}^{-7}M$

Answer 1

The correct option is A $5\times {10}^{-3}M$

For $Pb{I}_2,S=\sqrt[3]{3\frac{K_{sp}}{4}}=\sqrt[3]{3\frac{4\times {10}^{-9}}{4}}={10}^{-3}$
$Pb{I}_2\rightleftharpoons P{b}^{2+}+2I^{⊝}$
$\therefore \left[I^{⊝}\right]=2S=2\times {10}^{-3}M$
Now, for: $A{g}^{\oplus }+I^{⊝}⟶AgI$
mEq of $A{g}^{\oplus }=mEq$ of $I^{⊝}$
$N\times 10=2\times 106-3\times 25$
N or M of $AgN{O}_3=5\times {10}^{-3}$