Question

The number of moles of water of crystallization present per mole of the compound $A$ is:

• A. 10
• B. 5
• C. 7
• D. 8

Answer 1

The correct option is A 10

$A$ is borax or sodium tetraborate $\left(N{a}_2{B}_4{O}_7.10H_{2}O\right)$.
One mole of $A$ contains 10 moles of water of crystallization.

It is a white crystalline solid and is used in water softening. Its aqueous solution is alkaline due to the formation of sodium hydroxide upon hydrolysis of $N{a}_2{B}_4{O}_7$. The reaction is as follows:

$N{a}_2{B}_4{O}_7+7H_{2}O\rightleftharpoons 2NaOH+4H_{3}B{O}_3$

When borax is heated, it loses water and swells into a white mass which is $B$. The formula is $N{a}_2{B}_4{O}_7$. The reaction is as follows:
$N{a}_2{B}_4{O}_7.10H_{2}O\stackrel{heat}{\to }\underset{B}{N{a}_2{B}_4{O}_7}+10H_{2}O$

On further heating, $B$ forms glossy solid called borax glass and borax bead (compound $C$ or $B_2{O}_3$). The reaction is as follows:

$N{a}_2{B}_4{O}_7\stackrel{heat}{\to }2NaB{O}_2+\underset{C}{B_2{O}_3}$

When $C$ is heated with nickel oxide, a brown bead $D$ is obtained which is nickel metaborate $\left(Ni\right(B{O}_2{)}_2)$. The reaction is as follows:

$B_2{O}_3+NiO\to \underset{brownbead\left(D\right)}{\left(Ni\right(B{O}_2{)}_2)}$