Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

The aqueous solution of an inorganic compound \((\mathrm{X})\), gives a white precipitate with \(\mathrm{NH}_{4} \mathrm{OH}\), which does not dissolve in excess of \(\mathrm{NH}_{4} \mathrm{OH}\). This aqueous solution also gives white precipitate with \(\mathrm{AgNO}_{3}\) and the precipitate is soluble in dilute \(\mathrm{HNO}_{3} .\) Here \((\mathrm{X})\) is (a) \(\mathrm{AlCl}_{3}\) (b) \(\mathrm{AlBr}_{3}\) (c) AlN (d) \(\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}\)

Step-by-Step Solution

Answer

The compound \((\mathrm{X})\) is \(\mathrm{AlCl}_3\).

Step 1: Examine Reaction with NH4OH

The compound \((\mathrm{X})\) forms a white precipitate when reacted with \(\mathrm{NH}_{4} \mathrm{OH}\). This indicates the presence of metal cations that form insoluble hydroxides, such as \(\mathrm{Al}^{3+}\), which forms white \(\mathrm{Al(OH)}_3\) that does not dissolve in excess \(\mathrm{NH}_{4} \mathrm{OH}\).

Step 2: Examine Reaction with AgNO3

\((\mathrm{X})\) also forms a white precipitate with \(\mathrm{AgNO}_3\). Silver nitrate forms a white precipitate with halides like \(\mathrm{Cl}^-\) or \(\mathrm{Br}^-\), forming \(\mathrm{AgCl}\) or \(\mathrm{AgBr}\). The solubility of this precipitate in dilute \(\mathrm{HNO}_3\) suggests it is likely \(\mathrm{AgCl}\), as \(\mathrm{AgBr}\) does not dissolve in dilute \(\mathrm{HNO}_3\).

Step 3: Consider Compound Options

Among the options, \(\mathrm{AlCl}_3\) contains \(\mathrm{Cl}^-\) ions which form \(\mathrm{AgCl}\) with \(\mathrm{AgNO}_3\) and dissolve in \(\mathrm{HNO}_3\). The other options either don't contain \(\mathrm{Cl}^-\) ions or don't meet the solubility criteria observed.

Step 4: Conclusion

Based on the reactions observed with \(\mathrm{NH}_{4} \mathrm{OH}\) and \(\mathrm{AgNO}_3\), and the properties of the products, the compound is most likely \(\mathrm{AlCl}_3\). \(\mathrm{AlCl}_3\) fits all the given chemical behavior in the problem statement.

Key Concepts

White Precipitate

In chemistry, a white precipitate forms when certain compounds mix in solution, causing an insoluble solid to separate out and appear as a distinct, pale-colored solid.
These precipitates can indicate the presence of specific ions in a reaction. When a white precipitate forms, it's an indication that a chemical reaction has occurred between the reactants. This is a crucial observation in inorganic chemistry, often used to identify ions present in a solution.

• For instance, \( \mathrm{Al(OH)}_3 \) is a common white precipitate formed by aluminum cations when they react with hydroxide ions.
• Similarly, \( \mathrm{AgCl} \) and \( \mathrm{AgBr} \) are examples of white precipitates formed when silver ions react with chloride and bromide ions, respectively.

Observations of precipitate formation help chemists determine which ions are present in a sample, thus aiding in the identification of unknown compounds.

Metal Cations

Metal cations are positively charged ions formed when metals lose electrons. These cations are typically represented by the symbol of the element followed by a superscript representing the charge, such as \( \mathrm{Al}^{3+} \).
In inorganic chemistry, metal cations often create ionic bonds with negatively charged ions (anions), leading to the formation of salts and other compounds.

• Different metal cations have varying levels of solubility with different anions, which impacts their ability to form precipitates.
• Cations like \( \mathrm{Al}^{3+} \) are known for forming insoluble hydroxides, especially in the presence of hydroxide ions like those in \( \mathrm{NH_{4}OH} \).

The identification of metal cations through various chemical reactions is essential for understanding the composition and properties of a solution.

Halides

Halides are anions consisting of halogen atoms, such as chloride (\( \mathrm{Cl}^- \)), bromide (\( \mathrm{Br}^- \)), and iodide (\( \mathrm{I}^- \)). These are known for forming salts with metal cations, resulting in a variety of inorganic compounds common in chemistry.

• Halides readily react with silver nitrate to form distinct white precipitates—\( \mathrm{AgCl} \) for chloride and \( \mathrm{AgBr} \) for bromide.
• These reactions are a basis for qualitative analysis to identify different halide ions in solutions.

The detection of halide ions in solution aids in the identification of the composition and nature of inorganic compounds, relying heavily on their reactivity and solubility characteristics.

Solubility

Solubility is a key concept in chemistry, referring to the ability of a substance to dissolve in a solvent. It is influenced by temperature and the nature of the interacting substances.

• Inorganic compounds, like those containing metal cations and halides, vary in solubility depending on the ions involved.
• For example, \( \mathrm{AgCl} \) is typically insoluble in water but dissolves in dilute \( \mathrm{HNO}_3 \), while \( \mathrm{AgBr} \) does not.

Understanding solubility helps in predicting the behavior of compounds in aqueous solutions and is crucial in methods such as precipitation reactions for identifying components in mixtures.

Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of chemical bonds. In inorganic chemistry, these reactions are fundamental for exploring interactions between different ions and compounds.

• Precipitation reactions, as seen with \( \mathrm{X} \) forming a white precipitate with \( \mathrm{NH_4OH} \), highlight the reaction of ions to form new, often insoluble compounds.
• Double displacement reactions, such as those forming \( \mathrm{AgCl} \) from \( \mathrm{AgNO_3} \) and \( \mathrm{Cl}^- \), are critical for identifying unknown solutions.

Chemical reactions in the context of inorganic chemistry provide insights into the composition, reactivity, and properties of materials, supporting the identification and analysis of compounds in various scientific and industrial applications.