Question

Chromium hydroxide on reaction with excess \[{{NaOH}}\] gives a soluble compound. Guess the molecular formula.
A. \[{{[Cr(OH}}{{{)}}_{{3}}}{{]}}\]
B. \[{{{[Cr(OH}}{{{)}}_{{4}}}{{]}}^{{ - }}}\]
C. \[{{{[Cr(OH}}{{{)}}_{{5}}}{{]}}^{{{2 - }}}}\]
D. \[{{{[Cr(OH}}{{{)}}_{{6}}}{{]}}^{{{3 - }}}}\]

Answer 1

Hint: The reaction of chromium hydroxide with excess of \[{{NaOH}}\] is an oxidation reaction. In this reaction, chromium having oxidation state three will lose an electron to form chromium of oxidation state four to form a coloured compound.

Complete answer:
> Chromium being a d- block element has the ability to attain more than one valency and this is because the s and p orbitals could take part in a reaction and will donate electrons.
> The most common oxidation states or valencies of chromium includes +3 and +2. There also exists the +6 oxidation state.
> The reaction between chromium hydroxide and \[{{NaOH}}\] is a precipitation reaction. But chromium hydroxide dissolves only to a small extent but excess ammonia dissolves the precipitate completely.
> The reaction between \[{{NaOH}}\] and \[{{[Cr(OH}}{{{)}}_{{3}}}{{]}}\] can be shown as:
\[{{C}}{{{r}}^{{{3 + }}}}{{(aq)}}\;{{ + 3O}}{{{H}}^{{ - }}}{{(aq)}} \to {{Cr(OH}}{{{)}}_{{3}}}{{(s)}}\]
\[{{{[Cr(}}{{{H}}_{{2}}}{{O}}{{{)}}_{{6}}}{{]}}^{{{3 + }}}}\xrightarrow{{{{NaOH}}}}{{ [Cr(}}{{{H}}_{{2}}}{{O}}{{{)}}_{{3}}}{{{(OH)}}_{{3}}}{{]}}\xrightarrow{{{{NaOH}}}}{{ [Cr(OH}}{{{)}}_{{6}}}{{{]}}^{{{ - 3}}}}\]
> The reaction can be explained in words as the reaction between chromium hydroxide and excess sodium hydroxide produces a green solution of hexahydroxy chromate (\[{{III}}\]) ions. Here the hexaaquachromium \[{{(II)}}\] ions are converted to hexahydroxy chromate \[{{(III)}}\]ions. We will get the precipitate in a good amount if we boil the solution.

So, the correct answer is Option D.

Additional information:
> d- block elements in the periodic table are those elements whose d subshell is getting filled. They show different characteristics like variable valency, colour etc. The colour they show is formed because of the excitation of electrons.

Note: Variable or multiple valency is shown by chromium because of the participation of the electrons in their s and p subshells. They form complexes with different other compounds because of this property. Most of the coordinate compounds are formed by d-block elements.