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

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.