Question

The mineral colemanite is finely powdered and boiled with sodium carbonate solution. The precipitate of $CaC{O_3}$ and other two products $X$ and $Y$ are formed. On passing a current of $C{O_2}$, $Y$ produces $X$. What is the molecular formula of colemanite?
(A) $C{a_2}{B_6}{O_{11}}$
(B) $Ca{B_6}{O_{10}}$
(C) $C{a_3}{B_6}{O_{13}}$
(D) $C{a_2}{B_8}{O_{11}}$

Answer 1

Colemanite is a naturally occurring mineral found in the mines, it is an ore of boron. The chemical formula known as Colemanite is $C{a_2}{B_6}{O_{11}} \cdot 5{H_2}O$. It’s a colorless crystal and is brittle. Colemanite was the most important ore before the discovery of Kernite. It is also used for manufacturing heat resistant glass and has plenty of other uses.

Complete step by step answer:
There are over 200 minerals containing boron but only colemanite has commercial importance.
It is composed of 50 percent boron trioxide.
Boron trioxide is one of the oxides of boron and is used as an insecticide and also as a fluxing agent in enamels and glasses.
The reaction between colemanite and sodium carbonate solution is given as
$C{a_2}{B_6}{O_{11}} + N{a_2}C{O_3} \to N{a_2}{B_4}{O_7} + NaB{O_2} + CaC{O_3} \downarrow$
$CaC{O_3}$( calcium carbonate ) is a precipitate.
Here, $X$is $N{a_2}{B_4}{O_7}$ and $Y$is $NaB{O_2}$
$N{a_2}{B_4}{O_7}$ is Sodium Tetraborate commonly called borax.
$NaB{O_2}$ is called sodium metaborate.
As given in the question, when carbon dioxide is passed through sodium metaborate, sodium tetraborate is produced.
The reaction between sodium metaborate and carbon dioxide is given as
$NaB{O_2} + C{O_2} \to N{a_2}{B_4}{O_7} + N{a_2}C{O_3}$
sodium carbonate is the precipitate.
The molecular formula of colemanite is $C{a_2}{B_6}{O_{11}}$

So, the correct answer is option A.

Note: Borax obtained from the above reaction is a highly poisonous substance and is also officially banned in many countries. It can kill insects such as ants and is also used in detergents to get rid of stains and mold.
The 5 molecules of water present in Colemanite $C{a_2}{B_6}{O_{11}} \cdot 5{H_2}O$ is the water of crystallization.
The water comes out from the crystal substance after applying heat. After the evaporation of the water of crystallization, the composition of the molecule changes.