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Titration of Borax: Tincal : Suhaga :

When Borax dissolved in water, it is subject to completely dissociate, and then, hydrolyzes to orthoboric acid (B(OH)3  and [B(OH)4]anions,  and resulting PH  of  solution about 9.13.
According to the equation:
Na2[B4O5(OH)4]8H2O(s)2Na+(aq) + B4O5(OH)42− (aq) + 8H2O(l)
B4O5(OH)42−(aq) + 5H2O(l) 4 B(OH)3(aq) + 2OH(aq)

The liberated B(OH)3(aq)  acts as a Lewis acid on OH ions from auto ionization of water, and hence, in equilibrium with water:
B(OH)3(aq)+2H2O(l) B(OH)4(aq) + H3O+(aq)

Therefore, one mole of borax reacts with two moles of strong acid solutions. This is  because when Borax dissolved in water Both B(OH)3(aq)  and B(OH)4(aq) are formed but only  B(OH)4(aq) react with acid.  
For example:
                Na2[B4O5(OH)4].8H2O(aq) + 2HCl(aq) 4B(OH)3(aq)+ 2NaCl (aq) + 5 H2O

ILLUSTRATIVE EXAMPLE: Aqueous solution of borax reacts with two moles of acids.
This is because of:
(A) Formation of 2mol of B(OH)3 only
(B) Formation of 2mol of Na[B(OH)4]¯ only
(C) Formation of 1mol each of B(OH)3 and Na[B(OH)4
(D) Formation of 2mol each of Na[B(OH)4]¯and B(OH)3 of which only Na[B(OH)4]¯ reacts with acid.
Answer: (D)
 Na2B4O7 + 7H2O 2B(OH)3 + 2Na[B(OH)4]
B(OH)3 or H3BO3 is an acid and does not react with acid. Hence Na[B(OH)4] reacts with acid.

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