Question

Formula of ferrous bisulphite is:
A. ${\text{F}}{{\text{e}}_{\text{2}}}{\left( {{\text{HS}}{{\text{O}}_{\text{3}}}} \right)_{\text{3}}}$
B. ${\text{Fe}}{\left( {{\text{HS}}{{\text{O}}_{\text{3}}}} \right)_2}$
C. ${\text{FeHS}}{{\text{O}}_{\text{3}}}$
D. ${\text{F}}{{\text{e}}_3}{\left( {{\text{HS}}{{\text{O}}_{\text{3}}}} \right)_2}$

Answer 1

Hint: The term ‘ferrous’ in chemistry indicates that iron is in +2 oxidation state. The term ‘bisulphite’ in chemistry is used to refer to the salt or ester of sulphurous acid which contain the monovalent anion ${\text{HS}}{{\text{O}}_{\text{3}}}^ - $.

Complete step by step answer:
We need to find out the formula of the compound ferrous bisulphite. Let us check the given options one by one.
The first option has the compound ${\text{F}}{{\text{e}}_{\text{2}}}{\left( {{\text{HS}}{{\text{O}}_{\text{3}}}} \right)_{\text{3}}}$. Since each bisulphate anion carries only a single negative charge and there are 3 bisulphite anions, therefore, the total negative charge is -3. Also, there are two iron atoms. Let the oxidation state of iron be ‘x’. The sum of the total positive and negative charges will be equal to zero as the overall charge on the compound is zero. Then we have:
$
  2{\text{x}} + \left( { - 3} \right) = 0 \\
   \Rightarrow {\text{x}} = + \dfrac{3}{2} \\
 $
Thus, iron is not in +2 oxidation state in the compound ${\text{F}}{{\text{e}}_{\text{2}}}{\left( {{\text{HS}}{{\text{O}}_{\text{3}}}} \right)_{\text{3}}}$. Hence, it is not the correct formula for ferrous bisulphite.
The second option has the compound ${\text{Fe}}{\left( {{\text{HS}}{{\text{O}}_{\text{3}}}} \right)_2}$. Since each bisulphate anion carries only a single negative charge and there are two bisulphite anions, therefore, the total negative charge is -2. Also, there is only one iron atom. Let the oxidation state of iron be ‘x’. The sum of the total positive and negative charges will be equal to zero as the overall charge on the compound is zero. Then we have:
$
  {\text{x}} + \left( { - 2} \right) = 0 \\
   \Rightarrow {\text{x}} = + 2 \\
 $
Thus, iron is in +2 oxidation state in the compound ${\text{Fe}}{\left( {{\text{HS}}{{\text{O}}_{\text{3}}}} \right)_2}$. Hence, it is the correct formula for ferrous bisulphite.
If we proceed in a similar way, we can see that iron is not in +2 oxidation state for the compound ${\text{FeHS}}{{\text{O}}_{\text{3}}}$.

$
  {\text{x}} + \left( { - 1} \right) = 0 \\
   \Rightarrow {\text{x}} = + 1 \\
 $
Same is in the case of ${\text{F}}{{\text{e}}_3}{\left( {{\text{HS}}{{\text{O}}_{\text{3}}}} \right)_2}$ .
$
  {\text{3x}} + \left( { - 2} \right) = 0 \\
   \Rightarrow {\text{x}} = + \dfrac{2}{3} \\
 $

Hence, the correct option is B.

Note:
Bisulphite has good reducing properties and so are used in precipitating gold from auric acid and also to reduce hexavalent chromium to trivalent chromium. It is also used in water chlorination to reduce chlorine which has bad effects on aquatic life.
Bisulphite is a weak acidic species and its conjugate base is the sulfite ion.
${\text{HS}}{{\text{O}}_{\text{3}}}^ - \rightleftharpoons {\text{S}}{{\text{O}}_{\text{3}}}^ - + {{\text{H}}^ + }$