Which is not true about borax?

• A

  It is a useful primary standard for titrating against acids
• Correct answer
  B

  One mole of borax contains 4 $\mathrm{B}-\mathrm{O}-\mathrm{B}$ bonds
• C

  Aqueous solution of borax can be used as buffer
• D

  It is made up of two triangular $\mathrm{B}{\mathrm{O}}_3$ units and two tetrahedral $\mathrm{B}{\mathrm{O}}_4$ units

Solution

Here only in option B is incorrect while rest other are correct.
In borax $\left(\mathrm{N}{\mathrm{a}}_2{\mathrm{B}}_4{\mathrm{O}}_7.10{\mathrm{H}}_2\mathrm{O}\right),$ among 10 water molecules 2 molecules are part of structure, i.e., exists $\mathrm{N}{\mathrm{a}}_2\left[{\mathrm{B}}_4{\mathrm{O}}_5\right(\mathrm{O}\mathrm{H}{)}_4].8{\mathrm{H}}_2\mathrm{O}$

$\mathrm{N}{\mathrm{a}}_2\left[{\mathrm{B}}_4{\mathrm{O}}_5\right(\mathrm{O}\mathrm{H}{)}_4].8{\mathrm{H}}_2\mathrm{O}+2\mathrm{H}\mathrm{C}\mathrm{l}\to 2\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}+4{\mathrm{H}}_3\mathrm{B}{\mathrm{O}}_3+5{\mathrm{H}}_2\mathrm{O}$
Methyl orange (pH = 3.7) is used to detect end point. Aqueous solution of borax acts as buffer, as borax is salt of strong base NaOH and weak acid ${\mathrm{H}}_3\mathrm{B}{\mathrm{O}}_3$ .

Hence, the correct answer is option: 'B'.