Question

The formula of iron (III) sulphate is:
[A]$F{e}_{3}C{O}_4$
[B]$F{e}_2{(S{O}_4)}_3$
[C]$F{e}_2{(C{O}_4)}_3$
[D]$FeC{O}_4$

Answer 1

Hint: The correct answer for the formula of iron (III) sulphate will be the compound with a sulphate group. The central metal atom, which is iron in this case, will have an oxidation number of +3 which we can understand from the name itself.

Complete Step by step solution: As we can understand from the name itself, Iron (III) sulphate will contain an iron atom and there will be sulphate ions around it.
The Roman numeral three beside the central metal ion depicts the oxidation number on the metal atom. Here, we have iron as the central metal ion and thus the oxidation number of iron in iron (III) sulphate will be three. We will check each option to find out the oxidation number of iron in each of the given compounds.
In the first option we have$F{e}_{3}C{O}_4$
Let the oxidation number of iron be ‘x’.
$\begin{array}{rl}& 3x+(-2)\times 4=0\\ & or,x=\frac{8}{3}\end{array}$
Therefore, we can write that two iron atoms will have a charge of +3 and one iron atom has a charge of +2. Therefore, it is not the correct answer.
Similarly for the second option, $F{e}_2{(S{O}_4)}_3$-
$\begin{array}{rl}& 2x+(-2)\times 3=0\\ & or,x=3\end{array}$
Therefore, the oxidation number on each iron atom is +3.
Similarly for the third and fourth option, the oxidation number of iron atoms is +3 and +2 respectively.
Therefore, the correct answer maybe$F{e}_2{(S{O}_4)}_3$or$F{e}_2{(C{O}_4)}_3$.
As we know sulphate group is$S{O_4}^{2-}$but in the third option,$F{e}_2{(C{O}_4)}_3$we have a carbonate group, which is$C{O_4}^{2-}$. Therefore this cannot be the correct answer. We can write that the formula of iron (III) sulphate is$F{e}_2{(S{O}_4)}_3$.

Therefore the correct answer is option [B] $F{e}_2{(S{O}_4)}_3$

Note: We should not confuse iron (III) sulphate with iron (II) sulphate which is $FeS{O}_4$. We can easily say here that the answer will be $F{e}_2{(S{O}_4)}_3$ because all the other options have a carbonate group present but the question is for sulphate and $F{e}_2{(S{O}_4)}_3$ is the only option available but if this is not the case, we can still easily find the correct formula by finding out the oxidation numbers.