Question

What is the formula of white vitriol?
(A) $ZnS{O_4} \cdot {H_2}O$
(B) $ZnS{O_4} \cdot 4{H_2}O$
(C) $ZnS{O_4} \cdot 6{H_2}O$
(D) $ZnS{O_4} \cdot 7{H_2}O$

Answer 1

You should know that it is used for preserving skins and wood, in the bleaching paper, as a mordant in calico printing and in medicine as an astringent, styptic and emetic.

Complete step by step answer:
- Generally, Zinc sulphate heptahydrate is commonly known as white vitriol. It is generally obtained by dissolving zinc, zinc oxide, zinc hydroxide, zinc carbonate in dilute ${H_2}S{O_4}$ . The solution of zinc sulphate on crystallization gives colorless crystals of $ZnS{O_4} \cdot 7{H_2}O$ .
- Generally, white vitriol in large is obtained by heating ZnS with a temperature below ${900^ \circ }C$
$ZnS + {O_2}\xrightarrow{{Below{\text{ 90}}{{\text{0}}^ \circ }C}}ZnS{O_4}$
- Crystalline $ZnS{O_4} \cdot 7{H_2}O$ is a colorless crystalline solid white anhydrous and is an efflorescent ( is the migration of a salt to the surface of a porous material, where it forms a coating) solid.
- As we know it is an efflorescent solid and effloresces when exposed in air to give monohydrate. As we can see the last ${H_2}$ molecule is lost at ${300^ \circ }C$.
$ZnS{O_4} \cdot 7{H_2}O\xrightarrow{{{\text{Exposed in air}}}}ZnS{O_4} \cdot {H_2}O + 6{H_2}O$
$ZnS{O_4} \cdot {H_2}O\xrightarrow{{{{300}^ \circ }C}}2ZnO + S{O_2} + {H_2}O$
- The anhydrous zinc sulphate on heating above ${900^ \circ }C$ decomposes forming ZnO.
$2ZnS{O_{4(aq)}}\xrightarrow{{{{900}^ \circ }C}}2ZnO + S{O_2} + {O_2}$
- $ZnS{O_4}$ gives off white precipitate of $Zn{(OH)_2}$ with NaOH which dissolves on adding excess NaOH forming sodium zincate.
$ZnS{O_4} + 2NaOH \to Zn{(OH)_2} + N{a_2}S{O_4}$
$Zn{(OH)_2} + 2NaOH \to N{a_2}Zn{O_2} + 2{H_2}O$
Therefore, the formula of white vitriol is $ZnS{O_4}$.

- Hence, option D is the required answer.

Note: Remember that breathing zinc sulphate irritates the respiratory tract, causes nausea, vomiting, and stomach ache and can cause metallic taste in mouth, and death and also exposure to skin can damage the skin to ulcers.