Problem 135 - A Complete Resource Book in Chemistry for JEE Main

Question

Select the incorrect statement(s): (a) Milk of magnesia is an aqueous solution of \(\mathrm{Mg}(\mathrm{OH})_{2}\) (b) \(\mathrm{KOH}\) is a weaker base than \(\mathrm{NaOH}\) (c) \(\mathrm{CaO}_{2}\) is less stable than \(\mathrm{MgO}_{2}\) (d) \(\mathrm{Mg}^{2+}\) ions are precipitated with the addition of \(\mathrm{NH}_{4} \mathrm{OH}\) in the presence of \(\mathrm{NH}_{4} \mathrm{Cl}\)

Step-by-Step Solution

Answer

The incorrect statements are (c) and (d).

Step 1: Analyze Statement (a)

Statement (a) claims that milk of magnesia is an aqueous solution of \(\mathrm{Mg}(\mathrm{OH})_{2}\). Milk of magnesia is indeed a suspension of \(\mathrm{Mg}(\mathrm{OH})_{2}\) in water, which verifies this statement as correct.

Step 2: Analyze Statement (b)

Statement (b) suggests that \(\mathrm{KOH}\) is a weaker base than \(\mathrm{NaOH}\). Both \(\mathrm{KOH}\) and \(\mathrm{NaOH}\) are strong bases, but \(\mathrm{KOH}\) is generally considered to be slightly less strong than \(\mathrm{NaOH}\) in terms of dissociation in water. This statement is correct.

Step 3: Analyze Statement (c)

Statement (c) indicates that \(\mathrm{CaO}_{2}\) is less stable than \(\mathrm{MgO}_{2}\). However, \(\mathrm{CaO}_{2}\) refers to perchlorates which can be unstable, whereas \(\mathrm{MgO}_{2}\) has more stability. This statement is incorrect because typically \(\mathrm{MgO}\), not \(\mathrm{MgO}_{2}\), is stable. The confusion might stem from the erroneous compound notation.

Step 4: Analyze Statement (d)

Statement (d) suggests that \(\mathrm{Mg}^{2+}\) ions are precipitated with the addition of \(\mathrm{NH}_{4}\mathrm{OH}\) in the presence of \(\mathrm{NH}_{4}\mathrm{Cl}\). In this situation, magnesium ions usually do not precipitate in the presence of ammonium chloride because the complex formation with \(\mathrm{NH}_{4}^{+}\) reduces the concentration of free \(\mathrm{Mg}^{2+}\) ions. Hence, this statement is incorrect.

Key Concepts

Bases

In chemistry, bases are substances that can accept hydrogen ions (protons) or more generally, donate electron pairs. They play a crucial role in neutralizing acids and are often characterized by a bitter taste and slippery feel.

• Common bases include hydroxides such as sodium hydroxide (NaOH) and potassium hydroxide (KOH).
• Both NaOH and KOH are considered strong bases, meaning they disassociate completely in water, releasing OH^- ions.
• The strength of a base is determined by its ability to dissociate in water. KOH, though a strong base, is often slightly less effective in dissociation compared to NaOH.

Understanding the properties of bases helps in gauging their reactivity and their role in various chemical reactions and industrial applications.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions combine to form a solid product called a precipitate. This process plays a vital role in chemical analysis, water treatment, and material science.

• A classic example is the reaction of silver nitrate with sodium chloride to form silver chloride, a white precipitate.
• The formation of a precipitate depends on the solubility of the product formed; if the product is less soluble in water, it will precipitate out.
• In many cases, specific ions interact in solution to form insoluble compounds. For example, when magnesium ions (Mg^{2+}) encounter certain anions, they might form a precipitate if conditions permit.

Understanding which species form a precipitate allows chemists to predict the outcomes of reactions and manipulate them for desired products.

Ionic Compounds

Ionic compounds are formed when atoms transfer electrons, resulting in the formation of positive and negative ions. These ions attract each other to form a neutral compound.

• They usually consist of metals and nonmetals, such as the sodium chloride (NaCl), where sodium donates an electron to chlorine.
• These compounds form structured crystal lattices that contribute to many of their properties, such as high melting and boiling points.
• Ionic compounds typically dissolve in water, dissociating into their respective ions, making them conductive in solution.

The understanding of ionic compounds is essential as they are prevalent in many chemical processes and products in everyday life.

Chemical Stability

Chemical stability refers to the stability of a substance when it undergoes a chemical change or remains resistant to decomposition or reaction. Stability is influenced by factors such as molecular structure, environmental conditions, and the types of reactions a substance is involved in.

• Stable compounds tend not to react easily with other substances. This makes them useful as building blocks in various applications.
• MgO is typically seen as stable, but the mention of MgO_2 suggests a need to clarify: real stability comparisons are relevant for recognized compounds, such as MgO vs. magnesium peroxide life analogues.
• For atmospheric or thermal stability, compounds exhibiting less reactivity and decomposition possibility are preferred.

Understanding chemical stability is crucial for predicting compound behavior in different environments, impacting synthetic and analytical chemistry fields.