Question

In this experiment you are directed to add a limited amount of NaOH (aq) and then an excess amount. A similar strategy will be used with the addition of NH3(aq). Why? Group of answer choices Both NaOH and NH3 are weak bases. Both NaOH and NH3 are strong bases. Initially a metal hydroxide may form which becomes soluble when more base is added and the metal complex forms. Insoluble metal complexes are first formed which then are replaced by insoluble metal hydroxide precipitates. Initially a complex ion may form which becomes insoluble when more base is added and the metal hydroxide precipitate forms.

Answer 1

The reason for adding a limited amount and then an excess amount is that initially a metal hydroxide may form which becomes soluble when more base is added and the metal complex forms.

In qualitative analysis is a common to add the base in drops and then in excess. When added in drops, the metal hydroxide is formed. This metal hydroxide is often insoluble.

After this metal hydroxide is formed, the base could be added in excess such that the metal hydroxide dissolves in the excess base by forming a complex.

For instance;

CuCl2(aq) + 2NaOH(aq)  -------> Cu(OH)2(s) + 2NaCl(aq)

Cu(OH)2(s) + 2OH^-(aq) -------> [Cu(OH)4]^2+(aq)

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