Question

On the addition of mineral acid to an aqueous solution of borax, the following compound is formed:
A.Boron hydride
B.Ortho-boric acid
C.Meta-boric acid
D.Pyro-boric acid

Answer 1

Borax, which is also called sodium borate, sodium tetraborate, or disodium tetraborate is a compound with molecular formula $\text{N}{{\text{a}}_{\text{2}}}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}\text{.10}{{\text{H}}_{\text{2}}}\text{O}$. Borax is commonly used in detergents, enamels, and cosmetic products.

Complete step by step answer:
When borax reacts with any inorganic mineral acid like hydrochloric acid, sulphuric acid, nitric acid, or phosphoric acid, then it is easily converted to boric acid and other borates which have many applications. When it reacts with hydrochloric acid then it results in the formation of the “orthoboric acid” and the reaction may be written as follows:
$\text{N}{{\text{a}}_{\text{2}}}{{\text{B}}_{\text{4}}}{{\text{O}}_{\text{7}}}\text{.10}{{\text{H}}_{\text{2}}}\text{O + 2HCl}\to \text{4}{{\text{H}}_{\text{3}}}\text{B}{{\text{O}}_{\text{3}}}\text{+ 2NaCl + 5}{{\text{H}}_{\text{2}}}\text{O}$
The borax decahydrate is sufficiently stable and used as a “primary standard” in acid-base titrimetry.
When the ortho-boric acid is dissolved in boiling water and then heated above$\text{170}{{\text{ }}^{\text{0}}}\text{C}$, then it dehydrates to form the “Meta-boric Acid”. It is a white cubic crystalline solid that is slightly soluble in water. Metaboric acid melts at about$\text{236}{{\text{ }}^{\text{0}}}\text{C}$ and then when it is heated above $\text{300}{{\text{ }}^{\text{0}}}\text{C}$ then it forms the “Tetraboric acid” which is also known as the “pyroboric acid”.

Hence, the correct answer for the above question is option B, Ortho-boric acid.

Note:
The borax salt has a number of uses that can be listed as follows:
As a pH buffer – borate anions are used in chemical laboratories to make buffer solutions.
Water softening agent – though borax itself alone does not have high affinity for the hardness cations, it has been used a lot for that purpose though.
Flux – a mixture of borax and ammonium chloride used as flux when welding iron and steel.