Question

State two relevant observations:

ii. Ammonium hydroxide solution is added to zinc(II) nitrate solution in minimum quantities and then in excess.

Answer 1

The observations are as follows:

When ammonium hydroxide $\left({\mathrm{NH}}_4\mathrm{OH}\right)$ is added to zinc(II) nitrate solution $\left(\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2\right)$,then a white colored gelatinous precipitate zinc hydroxide $\left(\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\right)$ is formed.

• The reaction can be represented as:

$$\begin{gathered} \mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)+2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to 2{\mathrm{NH}}_4{\mathrm{NO}}_3\left(\mathrm{aq}\right)+\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\downarrow \\ \mathrm{White}\mathrm{Gelatinous}\mathrm{ppt}. \end{gathered}$$

This precipitate is soluble in excess ammonium hydroxide solution. This is due to the formation of $\left[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4\right]{\left(\mathrm{OH}\right)}_2$ .

• The reaction can be represented as:

$$\begin{gathered} \mathrm{Zn}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)+4{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to \left[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4\right]{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right) \\ \mathrm{Colorless},\mathrm{soluble} \end{gathered}$$