Question

Which of the following pairs of ions would be expected to precipitate when dilute solutions are mixed?
(A) \[N{a^ + },S{O_3}^{2 - }\]
(B) \[N{H_4}^ + ,C{O_3}^{2 - }\]
(C) \[N{a^ + },{S^{2 - }}\]
(D) \[F{e^{2 + }},PO_4^{3 - }\]

Answer 1

Hint: Positively charged metals and negatively charged non-metals combine to form an ionic compound. These ionic compounds may or may not be soluble in dilute solutions. Dilute solutions mainly have water in excess amount and water is polar in nature so polar compounds must dissolve in it according to the dissolves like principle. But some other factors like hydration and lattice energy also contribute to the insolubility of ionic compounds in water.

Complete Step by Step Solution:
Option A: \[N{a^ + },S{O_3}^{2 - }\]combine to form \[N{a_2}S{O_3}\]. Sodium sulphite is a water soluble white crystalline compound which is soluble in water. It dissolves in water but remains as such in ammonia and chlorine. It is slightly alkaline in nature. Thus, it is incorrect to say that it precipitates when dilute solutions are mixed.

Option B: \[N{H_4}^ + ,C{O_3}^{2 - }\]combines to form \[{(N{H_4})_2}C{O_3}\]. All the carbonates are water soluble compounds.
 ammonium carbonate is soluble in water but decomposes to yield ammonia, carbon dioxide, and water when added to hot water.

Option C: \[N{a^ + },{S^{2 - }}\]combines to form \[N{a_2}S\]. It is also a water-soluble species and gives rise to solutions of very high alkalinity.

Option D: \[F{e^{2 + }},PO_4^{3 - }\] combines to form \[F{e_3}{(PO_4^{})_2}\]. Ferric phosphate is a silverish colour compound which changes to a reddish-brown precipitate when dissolved in a dilute solution. This is because of formation of hydrated oxides.
Thus, the correct answer is D.

Note: High hydration enthalpy and low lattice energy favours solubility of a compound. Carbonates are very large in size because they are made of more than one atom with two negative charges. This increases the hydration energy which enhances its solubility in water. Hence, mostly carbonates are soluble.