The first row transition metal (M) that does not liberate H2 gas from dilute HCl is Copper (Cu), as it has a positive reduction potential (ECu2+/Cu∘=+0.34 V).
When 1 mol of CuSO4 reacts with excess aqueous KCN, it first forms Cu(CN)2, which is unstable and decomposes to CuCN and cyanogen gas (CN)2.
2CuSO4+4KCN→2Cu(CN)2+2K2SO4
2Cu(CN)2→2CuCN+(CN)2
The CuCN then dissolves in excess KCN to form a highly stable soluble complex, potassium tetracyanocuprate(I):
CuCN+3KCN→K3[Cu(CN)4]
The complex [Cu(CN)4]3− is so stable that its dissociation constant is extremely low. When H2S gas is passed through this solution, the concentration of Cu+ ions is insufficient to exceed the solubility product of Cu2S.
Therefore, no precipitate of metal sulphide is formed.
The amount of MS (or M2S) formed is 0 mol.