The number of moles of water of crystallization present per mole of the compound A is:
A
10
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B
5
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C
7
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D
8
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Solution
The correct option is A 10 A is borax or sodium tetraborate (Na2B4O7.10H2O). 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 Na2B4O7. The reaction is as follows:
Na2B4O7+7H2O⇌2NaOH+4H3BO3
When borax is heated, it loses water and swells into a white mass which is B. The formula is Na2B4O7. The reaction is as follows: Na2B4O7.10H2Oheat−−→Na2B4O7B+10H2O
On further heating, B forms glossy solid called borax glass and borax bead (compound C or B2O3). The reaction is as follows:
Na2B4O7heat−−→2NaBO2+B2O3C
When C is heated with nickel oxide, a brown bead D is obtained which is nickel metaborate (Ni(BO2)2). The reaction is as follows: