Question

Identify the cation in each of the following cases:
Sodium hydroxide solution when added to solution ' A' gives reddish-brown precipitate.

Answer 1

$\text{'}A\text{'}$ is ${\mathrm{Fe}}^{3+}$

The reaction of NaOH with ${\mathrm{Fe}}^{3+}$produces a reddish-brown precipitate of Ferric hydroxide, i.e. $\mathrm{Fe}(\mathrm{OH}{)}_3$.

$$\begin{gathered} \mathrm{Fe}{\left({\mathrm{NO}}_3\right)}_{3\left(\mathrm{aq}\right)}+3{\mathrm{NaOH}}_{(\mathrm{aql}}\to \mathrm{Fe}(\mathrm{OH}{)}_{3(\mathrm{s})}\downarrow +3{\mathrm{NaNO}}_{3\left(\mathrm{aq}\right)} \\ {\mathrm{Fe}}_{\left(\mathrm{aq}\right)}^{3+}+3{\mathrm{NO}}_{3\left(\mathrm{aq}\right)}^{-}+3{\mathrm{Na}}_{\left(\mathrm{aq}\right)}^{+}+3{\mathrm{OH}}_{\left(\mathrm{aq}\right)}^{-}\to \mathrm{Fe}(\mathrm{OH}{)}_{3(\mathrm{s})}\downarrow +3{\mathrm{Na}}_{\left(\mathrm{aq}\right)}^{+}++3{\mathrm{NO}}_{3\left(\mathrm{aq}\right)}^{-} \\ \mathrm{Reddish}-\mathrm{brown} \\ \mathrm{ppt} \end{gathered}$$