Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{Mg}\) is not precipitated in \(\mathrm{V}\) group because (a) \(\mathrm{MgCO}_{3}\) is soluble in \(\mathrm{NH}_{4} \mathrm{Cl}\) (b) \(\mathrm{MgCO}_{3}\) is soluble in \(\mathrm{NH}_{4} \mathrm{OH}\) (c) \(\mathrm{MgCO}_{3}\) is soluble in water (d) none

Step-by-Step Solution

Answer

The answer is (a) \(\mathrm{MgCO}_{3}\) is soluble in \(\mathrm{NH}_{4} \mathrm{Cl}\).

Step 1: Understanding Precipitation

To solve this problem, we first need to understand why a compound is or isn't precipitated in chemical processes. Precipitation occurs when a compound becomes insoluble in the given solution and forms a solid. For Group V of the periodic table, this typically involves reactions that form insoluble hydroxides or carbonates.

Step 2: Investigating Solubility

The problem states that magnesium ions (\(\text{Mg}^{2+}\)) do not precipitate out in Group V precipitation tests. Group V typically precipitates carbonates in the presence of ammonium chloride (\(\text{NH}_4\text{Cl}\)) and ammonium hydroxide (\(\text{NH}_4\text{OH}\)). Magnesium carbonate (\(\text{MgCO}_3\)) could be a potential precipitate.

Step 3: Analyzing Solubility of Magnesium Carbonate

Now we must evaluate the solubility of \(\text{MgCO}_3\): - \(\text{NH}_4\text{Cl}\) in solution releases \(\text{NH}_4^+\), which increases acidity, and can sometimes keep \(\text{MgCO}_3\) soluble.- \(\text{NH}_4\text{OH}\) controls the \(\text{OH}^-\) ion concentration, influencing what precipitates.- If \(\text{MgCO}_3\) is soluble in water or either reagent, it won't precipitate.

Step 4: Considering Solubility Effects

Given that \(\text{MgCO}_3\) should not form a solid in Group V, the most logical explanation is that it remains soluble in the test conditions. Options (a) \(\text{MgCO}_3\) is soluble in \(\text{NH}_4\text{Cl}\) aligns well with our understanding as Group V uses these conditions, thus keeping \(\text{MgCO}_3\) from precipitating.

Step 5: Determining the Answer

From the above analysis, the solvation of \(\text{MgCO}_3\) in \(\text{NH}_4\text{Cl}\) prevents its precipitation, as the inclusion of \(\text{NH}_4\text{Cl}\) often dissolves more compounds that might otherwise precipitate. Therefore, the correct answer is (a).

Key Concepts

Magnesium Carbonate Solubility

Magnesium carbonate (\(\text{MgCO}_3\)) is a fascinating compound when it comes to its solubility properties. In chemistry, solubility determines whether a substance will dissolve or precipitate when introduced to a solution.
When studying \(\text{MgCO}_3\), it's essential to recognize that it is not very soluble in water. This typically results in magnesium carbonate often forming a precipitate, or solid, in aqueous solutions.
However, the presence of certain ions can alter this behavior. For example, the addition of specific ions such as ammonium (\(\text{NH}_4^+\)) can influence solubility, which is particularly relevant in group precipitation tests.

Ammonium Chloride Effect

The effect of ammonium chloride (\(\text{NH}_4\text{Cl}\)) on \(\text{MgCO}_3\) solubility is crucial to understand why it does not precipitate in Group V tests. When ammonium chloride is in solution, it dissociates into \(\text{NH}_4^+\) and \(\text{Cl}^-\) ions.
The presence of \(\text{NH}_4^+\) increases the acidity of the solution. This is because \(\text{NH}_4^+\) ions can lead to the formation of \(\text{NH}_3\) and water, releasing \(\text{H}^+\) ions into the solution. These added hydrogen ions increase the solubility of magnesium carbonate, thus preventing it from forming a solid.
Understanding this dynamic is vital for predicting the behavior of compounds during precipitation reactions in a laboratory setting.

Chemical Precipitation Process

The chemical precipitation process involves transforming dissolved substances into solid, insoluble forms, often through chemical reactions. During a chemical analysis, identifying whether a substance will precipitate can be crucial for separating and analyzing components.
Typically, precipitation occurs when the solubility product of the compound is exceeded. But in the case of \(\text{MgCO}_3\) during Group V tests, its solubility in the presence of \(\text{NH}_4\text{Cl}\) prevents it from precipitating.
This highlights that the presence of certain chemicals can play a pivotal role in solubility changes. For compounds like \(\text{MgCO}_3\), the controlled addition of substances such as \(\text{NH}_4\text{Cl}\) can be used strategically to manage their solubility, illustrating the delicate balance of conditions required for chemical precipitation.