Ionic EquilibriumHard

Question

Unexposed silver halides are removed from photographic film when they react with sodium thiosulphate to form the complex ion Ag(S2O3)23−. What amount of Na2S2O3 is needed to prepare 1 L of a solution that dissolves 0.1 moles of AgBr by the formation of Ag(S2O3)23−? Ksp of AgBr = 4.0 × 10–13 and Kf of Ag(S2O3)23− = 1.6 × 1012.

Options

A.3.25 g
B.3.25 moles
C.0.325 moles
D.6.5 moles

Solution

$AgBr(s) + 2S_{2}O_{3}^{2 -} \rightleftharpoons Ag\left( S_{2}O_{3} \right)_{2}^{3 -} + Br^{-}$

0.1 a M 0 0

Eqn/final 0 a – 0.2 0.1 0.1

$K_{eq} = 4 \times 10^{- 13} \times 1.6 \times 10^{12} = \frac{0.1 \times 0.1}{(a - 0.2)^{2}} \Rightarrow a = 0.325$

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