Question

Sodium carbonate cannot be used in place of ammonium carbonate for the identification of the fifth group radicals. This is because the (1) sodium ions will interfere in the detection of the fifth group radicals (2) concentration of carbonate ions is very low (3) sodium will react with acidic radicals (4) magnesium will be precipitated

Short answer

Option (4): magnesium will be precipitated.

Step-by-step solution

- Understand the Context

The problem requires identifying why sodium carbonate cannot be used instead of ammonium carbonate for detecting fifth group radicals in qualitative analysis.

- Analyze the Options

Review each given option to evaluate its validity: (1) Sodium ions will interfere in the detection of the fifth group radicals (2) Concentration of carbonate ions is very low (3) Sodium will react with acidic radicals (4) Magnesium will be precipitated

- Evaluate Option 1

Sodium ions do not generally interfere in the precipitation reactions used to identify the fifth group radicals since these tests are based on the precipitation of specific metal cations.

- Evaluate Option 2

The concentration of carbonate ions in sodium carbonate solution is comparable to that in ammonium carbonate and thus can precipitate similar carbonate salts.

- Evaluate Option 3

Sodium ions do not specifically react with acidic radicals in a way that would impede the fifth group identifications.

- Evaluate Option 4

In the presence of sodium carbonate, magnesium will precipitate as magnesium carbonate, which is not preferable during the identification of fifth group radicals as it can create hindrance.

- Confirm the Answer

Based on the evaluations, option (4) correctly identifies why sodium carbonate cannot be used; it leads to the precipitation of magnesium.

Key concepts

Fifth Group Radicals

Fifth group radicals refer to a specific category of cations in qualitative analysis. In this context, 'radicals' is another term for ions. These cations include magnesium, calcium, strontium, and barium. During qualitative analysis, specific procedures are often used to detect these ions. Identifying fifth group radicals typically involves precipitation reactions. This is done by adding reagents that will form insoluble compounds with the respective ions. The key is to select reagents that precipitate only the desired ions without affecting others, making detection easier and more accurate.

Sodium Carbonate

Sodium carbonate (Na₂CO₃) is a common reagent in chemical analysis. It's a white, water-soluble powder with strong alkaline properties. Sodium carbonate can react with various cations to form precipitates. This makes it useful in many chemical reactions. In the context of fifth group radicals, sodium carbonate is not ideal. Here's why: sodium ions (Na⁺) don't play a significant role in the interference but magnesium ions (Mg²⁺) present in the mixture will react with sodium carbonate to form magnesium carbonate. This undesired precipitation can mask the detection of other fifth group radicals. Additionally, since sodium carbonate can precipitate fifth-group cations, it causes unwanted results in the test.

Ammonium Carbonate

Ammonium carbonate ((NH₄)₂CO₃) is another reagent used in qualitative analysis. It is preferred over sodium carbonate for identifying fifth group radicals. Ammonium carbonate is more effective because it does not precipitate magnesium ions. Instead, magnesium remains soluble as ammonium magnesium carbonate complex. This selectivity is crucial during the qualitative detection process. Ammonium ions from ammonium carbonate do not form interfering precipitates with fifth group cations. This allows for clearer and more accurate identification of magnesium, calcium, strontium, and barium ions.

Precipitation Reactions

Precipitation reactions are fundamental in the qualitative analysis of ions. They involve the formation of a solid (precipitate) from a solution when specific reagents are added. For example, to identify fifth group radicals, we add reagents that form insoluble compounds with these ions. If the solid forms, it indicates the presence of particular cations. Choosing the right reagent is key:

• An effective reagent must precipitate only the target ions.
• It should not cause precipitation of ions from other groups.
• It should result in clear and easily detectable precipitates.

For fifth group cations, ammonium carbonate is preferred. It selectively precipitates these ions without interfering with others such as magnesium. This ensures accurate identification in qualitative analysis. Understanding how precipitation reactions work and why specific reagents are chosen is crucial for accurate detection in qualitative analysis.