Question

Dissociation of $Fe{(OH)_3}$
will be least in:
A. 0.1M NaOH
B. 0.1M HCl
C. 0.1 M $Ba{(OH)_2}$
D. ${H_2}O$

Answer 1

This question can be solved by the common ion effect method, where the solubility of a weak electrolyte is decreased by adding a strong electrolyte solution which has a common ion.

Complete step by step answer:
-The common ion effect is the method to decrease the solubility of the ionic precipitate by adding soluble compounds which carry one ion in the same precipitate.
-The compound iron (III) hydroxide $Fe{(OH)_3}$ is a weak base therefore it is a weak electrolyte. It does not completely dissociate into its constituent ions when dissolved in water.
-The dissociation of iron (III) hydroxide $Fe{(OH)_3}$ is shown below.
$Fe{(OH)_3} \to F{e^{3 + }} + 3O{H^ - }$
-In this reaction, one mole of iron (III) hydroxide gives one mole of iron and three mole of hydroxide ions.
-As iron (III) hydroxide is a weak base, a strong base electrolyte is added which has a common ion to decrease its solubility.
-0.1M NaOH is a strong electrolyte, which completely dissociates into its constituent ions.
-The dissociation of NaOH is shown below.
$NaOH \to N{a^ + } + O{H^ - }$
-In this reaction, sodium hydroxide dissociates into sodium ion and hydroxide ion.
-The iron (III) hydroxide and sodium hydroxide have hydroxide ions in common. When a strong electrolyte solution is added in the solution of iron (III) hydroxide, it will decrease the dissociation of iron (III) hydroxide.
-Thus, the dissociation of iron (III) hydroxide $Fe{(OH)_3}$ will be least in 0.1 M NaOH.
Therefore, the correct option is A.

Note:
-The solution of 0.1M barium hydroxide $Ba{(OH)_2}$ is also a base electrolyte, but it is a weak electrolyte as it does not completely dissociate into its constituent ions.
-The dissociation of barium hydroxide is shown below.
$Ba{(OH)_2} \to B{a^{2 + }} + 2O{H^ - }$