Question

What is $X$ in the reaction? ​$N{a_2}{B_4}{O_7}\, + X \to {H_3}B{O_3}$
(A) Aqueous solution of NaOH
(B) Dilute nitric acid
(C) Con. ${H_2}S{O_4}$ or $HCl$
(D) Water

Answer 1

The chemical name of $N{a_2}{B_4}{O_7}$ is sodium tetraborate. It is commonly known as borax. Borax is a soft, colourless water-soluble compound. It exists as anhydrous and decahydrate salt forms. It is obtained naturally through recrystallization.

Complete step-by-step answer: The above-given reaction is an application of borax. It is used to prepare ${H_3}B{O_3}$ which is known as boric acid or Hydrogen Borate. Boric acid is a weak monobasic of boron. It exists as a white powder of colorless crystal that is soluble in water. Its solubility depends on temperature. It is also soluble in boiling water, but when it is heated above ${170^0}$, it gets dehydrated and forms metaboric acid which is a cubic crystalline and partially soluble in water, aniline and sparingly soluble in pyridine. The anion of boric acid is borate anion.

When borax is treated with a mineral acid like sulphuric acid or hydrochloric acid, it leads to the formation of Boric acid. The reaction with sulphuric acid will be:
$N{a_2}{B_4}{O_7}\, + {H_2}S{O_4} + 5{H_2}O \to 4{H_3}B{O_3} + N{a_2}S{O_4}$ and with Hydrochloric acid will be:
$N{a_2}{B_4}{O_7}\, + 2HCl + 5{H_2}O \to 4{H_3}B{O_3} + 2NaCl$
Hence, we can say that in the given reaction $X$ is Hydrochloric acid ($HCl$) or Sulphuric acid $({H_2}S{O_4})$.

Additional information: Borax is used in laundry and household cleaning products. The ions of borate are used to make buffers in chemical and biochemical laboratories. When borax is mixed with ammonium chloride it is used as a flux in iron and steel welding.

Therefore, option (C) is correct.

Note: The geometry of Boric acid is Trigonal Planar in which there are $3$ boron-oxygen single bonds. It is used in the manufacturing of textile fiberglass, flat panel displays. It is also used in the welding, jewellery industry, and as an antiseptic and antibacterial.