Question

1- why Pb2+ form a ppt with Cl- and if the ppt placed in hot water it would dissolve?

2- why when NH3(aq) add to the Al3+ it would form solid then if more NH3(aq) been add it would form complex ion?

3- why when NH3(aq) add to Mn2+ it would form solid even when extra NH3(aq) add it didn’t dissolve but it would dissolve with HNO3(aq)?

4- why Ni2+ didn’t form a ppt with H2S while Cu2+ form a black ppt?

I need answers for these questions

Answer 1

1. PbCl₂ dissolves in hot water.

PbCl₂(s) ⇌ Pb²⁺(aq)+ 2Cl^⁻(aq)

But you wouldn't have seen the PbCl₂ dissolve, because the other precipitate still remained.

2. Metals ions may react with NH₃ to form either insoluble precipitates or soluble complex ions. If a precipitate forms, it is the neutral hydroxide. For example, Al³⁺(aq) reacts with NH₃(aq) to form Al(OH)₃(s) which is a neutral hydroxide.

  Al³⁺(aq) + 3 NH₃(aq) + 3 H₂O(l) → Al(OH)₃(s) + 3 NH₄⁺(aq)

3. When NH₃(aq) add to Mn²⁺ it would form solid even when extra NH₃(aq) add it didn’t dissolve but it would dissolve with HNO₃(aq)​ as HNO₃ is strong oxdizing agent which dissolve the Mn²⁺

4.  Ni²⁺ didn’t form a ppt with H₂S while Cu²⁺ form a black ppt

As when the solution pass through H₂S gas from the Kipp's apparatus by pressing the nozzle.

Then Centrifuge and separate the precipitate.

Heat the black precipitate with minimum quantity (1-2 ml) of 50% HNO₃ in a tests tube - Precipitate dissolves. This Black precipitate dissolves in 50% HNO₃ either due to the formation of Pb(NO3)₂ or due to the formation of Cu(NO3)₂ and no black ppt of Ni²⁺ formed it formed when it Pass H₂S gas through the Ammonical solution.