Question

Sodium carbonate cannot be used in place of ammonium carbonate for the precipitation of the fifth group radicals because
(A) Sodium ions interfere with the detection of fifth group radicals
(B) Concentration of carbonate ions is very low
(C) Sodium will react with acidic radicals
(D) Magnesium will be precipitated

Answer 1

Hint: Precipitation is the process of changing a dissolved substance from a supersaturated solution to an insoluble solid in an aqueous solution. Precipitation refers to the produced solid. The chemical agent that initiates the precipitation in an inorganic chemical process is referred to as the precipitant. The term "supernate" or "supernatant" also refers to the clear liquid that remains on top of the precipitated or centrifuged solid phase.

Complete Step by Step Solution:
The methodical process of qualitative analysis uses precipitation reaction to eliminate cations sequentially from a mixture. The rationale for the separation of the cations is found in how they each respond differently to a group of common test reagents.

Magnesium (II), potassium (I) and sodium (I) are the cations that make up group 5. Hydrochloric acid (HCl), hydrogen sulphide, ammonium sulphide, and ammonium carbonate do not react with these cations. Similar to IVth group cations, magnesium reacts to generate the basic magnesium carbonate \[\mathbf{MgC}{{\mathbf{O}}_{\mathbf{3}}}\mathbf{.Mg}{{\left( \mathbf{OH} \right)}_{\mathbf{2}}}\].
\[{{\left( \text{N}{{\text{H}}_{\text{4}}} \right)}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\] causes group V cations to precipitate as carbonates. Because \[\text{N}{{\text{a}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\] will precipitate MgCO₃, we cannot substitute it for \[{{\left( \text{N}{{\text{H}}_{\text{4}}} \right)}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\]. Since \[\text{MgC}{{\text{O}}_{\text{3}}}\] has a high solubility product and when used with \[\text{N}{{\text{a}}_{\text{2}}}\text{C}{{\text{O}}_{\text{3}}}\], which totally ionises and supplies a high concentration of carbonate ions, precipitation results because the ionic product of \[\text{MgC}{{\text{O}}_{\text{3}}}\]exceeds its solubility product.
Hence option (D) is correct.

Note: When a chemical substance in the solid form is in chemical equilibrium with a solution of that molecule, a solubility equilibrium is a sort of dynamic equilibrium that develops. The solid may dissolve chemically reacting with another component of the solution, such as acid or alkali, or dissolving unaltered. A temperature-dependent solubility product that acts as an equilibrium constant identifies each solubility equilibrium. In many different situations, including those involving medicine and the environment, solubility equilibria are crucial.