Problem 1

(1.1) Show that 0.1 mol of

\ce{Tl2S}

dissolves in one litre of a 1 M solution of any strong monoprotic non-coordinating acid. (1.2) Show that 0.1 mol of

\ce{CuS}

dissolves in 1 M

\ce{HNO3}

but not in 1 M

\ce{HCl}

. Data:

E^0(\ce{S/S^{2-}}) = -0.48

E^0(\ce{NO3^-/NO}) = 0.96

V; p

K_a(\ce{H2S}) = 7

; p

K_a(\ce{HS^-}) = 13

;

K_{sp}(\ce{Tl2S}) = 1\times10^{-20}

;

K_{sp}(\ce{CuS}) = 1\times10^{-35}

; solubility of

\ce{NO}

in water (298 K):

2.53\times10^{-2}

mol dm

^{-3}

. Assume that

\ce{Cu^{2+}}

does not form stable complexes with

\ce{Cl^-}

Step 3 of 4: CuS resists non-oxidising acid

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[\ce{Cu^{2+}}][\ce{S^{2-}}] = 0.1 \times 10^{-21} = 10^{-22} > K_{sp}(\ce{CuS}) = 10^{-35}\ \Rightarrow\ \text{no dissolution in } \ce{HCl}

Analysis

The same speciation gives $[\ce{S^{2-}}] \approx 10^{-21}$ M, but the ion product $10^{-22}$ still exceeds $K_{sp}(\ce{CuS}) = 10^{-35}$ by thirteen orders of magnitude, so CuS cannot dissolve in 1 M HCl.