Question

Identify the formula of green mass.
A. $A{{l}_{2}}{{O}_{3}}.CoO$
B. $SnO.CoO$
C. $ZnO.CoO$
D. $MgO.CoO$

Answer 1

Hint: The green mass is also known as the Rinmann’s green or the cinnabar green. The formation of green mass involves decomposition reaction of the carbonate of respective elements.

Complete step-by-step answer: In the first option as we can see $A{{l}_{2}}{{O}_{3}}.CoO$ is present. This compound is also termed as cobalt aluminate or the Thenard’s blue. This compound is blue in colour and is not called green mass, so this option would be incorrect.
Now we will consider the compound $SnO.CoO$ which is also termed as cobalt stannate which is a compound having blue colour and so this would be an incorrect answer.
Now if we move to option C, which is $ZnO.CoO$ we know that this compound could be termed as cobalt zincate. It is prepared by the use of zinc carbonate which has the formula $ZnC{{O}_{3}}$ along with the oxide of the zinc which has the formula $ZnO$. Now we will see what happens in the reaction.
$ZnC{{O}_{3}}\to ZnO+C{{O}_{2}}$
We can see that the zinc carbonate undergoes decomposition and it breaks into zinc oxide with the release of carbon dioxide. Now, in the next step this zinc oxide will react with the sodium hydroxide with the formula $NaOH$ in order to form a sodium containing zinc oxide with the release of water. The reaction can be represented as,
$ZnO+NaOH\to N{{a}_{2}}Zn{{O}_{2}}+{{H}_{2}}O$
Now this sodium containing zinc oxide is also termed as sodium zincate which is represented by the formula $N{{a}_{2}}Zn{{O}_{2}}$ and this compound is kept aside and another cobalt containing compound is taken, which undergoes decomposition in order to form cobalt oxide. The reaction is represented below,
$2Co{{(N{{O}_{3}})}_{2}}\to 2CoO+4N{{O}_{2}}+{{H}_{2}}O$
Now, in the next step we take the zinc oxide and this cobalt oxide together on a charcoal cavity in for the formation of $ZnO.CoO$ which is also called the green mass.
And as for the option D, the cobalt oxide of magnesium is white in colour, so it would be the incorrect option.

Hence, the correct answer would be option C.

Note: The green mass is a compound which is made up by the combination of cobalt oxide and zinc oxide. Both of these oxides are obtained by decomposition reaction of their respective carbonates. The formula of green mass is $ZnO.CoO$.