Chemistry - Complete Pblock- Question with Solution | TestHub

In borax$\left({\text{Na}}_2{\text{B}}_4{\text{O}}_7·10{\text{H}}_2\text{O}\right)$, among 10 water molecules 2 molecules are part of structure, i.e., exists${\text{Na}}_2\left[{\text{B}}_4{\text{O}}_5{\left(\text{OH}\right)}_4\right]·8{\text{H}}_2\text{O}$



${\text{Na}}_2\left[{\text{B}}_4{\text{O}}_5{\left(\text{OH}\right)}_4\right]·8{\text{H}}_2\text{O}+2\text{HCl}\to 2\text{NaCl}+4{\text{H}}_3{\text{BO}}_3+5{\text{H}}_2\text{O}$

At end point pH is about 4 and, therefore, Methyl orange (works between pH = 3.1 - 4.4) is used to detect end point. Aq. solution of borax acts as buffer, as borax is salt of strong base NaOH and weak acid H₃BO₃.

${\text{B}}_4{\text{O}}_5{\left(\text{O}\text{H}\right)}_4^{2-}\left(\text{s}\right)+5{\text{H}}_2\text{O}\left(\text{l}\right)\rightleftharpoons \underset{\left(\text{w}\text{e}\text{a}\text{k}\text{a}\text{c}\text{i}\text{d}\right)}{2\text{B}{\left(\text{O}\text{H}\right)}_3\left(\text{a}\text{q}\right)}+\underset{\left(\text{s}\text{a}\text{l}\text{t}\right)}{2{\left[\text{B}{\left(\text{O}\text{H}\right)}_4\right]}^{-}\left(\text{a}\text{q}\right)}$