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

Statements (a), (b), and (c) are incorrect.

Step 1: Analyze Statement (a)

Statement (a) claims that milk of magnesia is an aqueous solution of \(\mathrm{Mg(OH)}_2\). Milk of magnesia indeed contains \(\mathrm{Mg(OH)}_2\), but it is not a solution; it is a suspension because \(\mathrm{Mg(OH)}_2\) is sparingly soluble in water. Hence, statement (a) is incorrect.

Step 2: Analyze Statement (b)

Statement (b) compares the basicity of \(\mathrm{KOH}\) and \(\mathrm{NaOH}\). Both \(\mathrm{KOH}\) and \(\mathrm{NaOH}\) are strong bases, but \(\mathrm{KOH}\) and \(\mathrm{NaOH}\) are approximately equally strong in dilute solutions. Therefore, this statement is incorrect as well.

Step 3: Analyze Statement (c)

Statement (c) suggests that \(\mathrm{CaO}_2\) is less stable than \(\mathrm{MgO}_2\). Actually, \(\mathrm{CaO}\) and \(\mathrm{MgO}\) exist commonly instead of \(\mathrm{CaO}_2\) and \(\mathrm{MgO}_2\). Hence, this statement about stability is not accurate and so it is incorrect.

Step 4: Analyze Statement (d)

Statement (d) claims that \(\mathrm{Mg}^{2+}\) ions are precipitated with the addition of \(\mathrm{NH_4OH}\) in the presence of \(\mathrm{NH_4Cl}\). This is a correct laboratory procedure for precipitating \(\mathrm{Mg(OH)}_2\) because \(\mathrm{NH_4Cl}\) suppresses the ionization of \(\mathrm{NH_4OH}\), allowing for the precipitation of \(\mathrm{Mg(OH)}_2\). Thus, statement (d) is correct.

Key Concepts

Milk of Magnesia

Milk of Magnesia is often mistaken for an aqueous solution, but it is actually a suspension. This is because it contains magnesium hydroxide, \( \mathrm{Mg(OH)}_2 \), which is only sparingly soluble in water. When mixed, \( \mathrm{Mg(OH)}_2 \) forms a cloudy mixture where fine particles are suspended throughout the liquid. Milk of Magnesia is commonly used for medicinal purposes, especially as an antacid to neutralize stomach acid and provide relief from indigestion.

• Composition: Mostly consists of magnesium hydroxide suspended in water.
• Uses: Primarily used to relieve digestive issues like acid indigestion and constipation.

In terms of chemistry, it's important to remember that suspensions, unlike true solutions, have solute particles that can settle over time. Hence, when you see a bottle of Milk of Magnesia, make sure to shake it well before use to redistribute the particles evenly.

Basicity comparison

When comparing the basicity of \( \mathrm{KOH} \) and \( \mathrm{NaOH} \), it is essential to understand that both are strong bases. This means both of them completely dissociate into their respective ions in solution. As a result, the hydroxide ions, \( \mathrm{OH}^- \), readily interact with hydrogen ions, \( \mathrm{H}^+ \), in solution, making them equally effective in neutralizing acids.

• Strong bases dissociate fully in water, meaning \( \mathrm{KOH} \) and \( \mathrm{NaOH} \) have similar functionalities.
• They are often used interchangeably in chemical experiments and industries due to this property.

While different sources might hint at slight differences in strength due to ionic radii or solubility, these differences are generally negligible in practical situations, especially in dilute solutions.

Stability of metal oxides

The stability of metal oxides, such as magnesium oxide (\( \mathrm{MgO} \)) and calcium oxide (\( \mathrm{CaO} \)), is an interesting topic to consider. In general, both \( \mathrm{MgO} \) and \( \mathrm{CaO} \) are highly stable compounds with significant uses in practical applications.

• Thermodynamic stability refers to how much energy is required to break a substance into its components. Both \( \mathrm{MgO} \) and \( \mathrm{CaO} \) are extremely stable, making them useful in high-temperature applications.
• These compounds are typically seen as basic oxides, reacting readily with water to form their respective hydroxides.

It is important to note that the popularity of using these oxides in various industries stems from their stability, not from terms mistakenly given in exercises like \( \mathrm{MgO}_2 \) and \( \mathrm{CaO}_2 \), which are not commonly existing forms.

Precipitation reactions

Precipitation reactions are a type of chemical reaction where two solutions together form an insoluble solid called a precipitate. In the context of magnesium ions (\( \mathrm{Mg}^{2+} \)) and their precipitation, the introduction of ammonium hydroxide, \( \mathrm{NH}_{4} \mathrm{OH} \), in the presence of ammonium chloride, \( \mathrm{NH}_{4} \mathrm{Cl} \), helps precipitate \( \mathrm{Mg(OH)}_2 \).

• The role of \( \mathrm{NH}_{4} \mathrm{Cl} \) is critical as it suppresses the ionization of \( \mathrm{NH}_{4} \mathrm{OH} \), allowing the \( \mathrm{Mg(OH)}_2 \) to precipitate instead of remaining dissolved.
• This is a common laboratory technique used to purify magnesium ions from a mixture by converting them into an easily removable solid form.

Such reactions are crucial in various chemical processes, particularly in analytical chemistry where they help in the identification and removal of certain ions from a solution.