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 2 of 4: Tl₂S dissolves

← Previous Next →

[\ce{Tl+}]^2[\ce{S^{2-}}] = (0.2)^2 \times 10^{-21} = 4\times10^{-23} < K_{sp}(\ce{Tl2S}) = 10^{-20}

Analysis

If 0.1 mol $\ce{Tl2S}$ dissolves per litre, $[\ce{Tl+}] = 0.2$ M and total sulphide $c = 0.1$ M. The ion product $4\times10^{-23}$ is below $K_{sp} = 10^{-20}$ , so dissolution in 1 M strong acid is spontaneous.