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

Hint: 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.
$$Ba(OH{)}_2\to B{a}^{2+}+2O{H}^{-}$$