Problem 164 - A Complete Resource Book in Chemistry for JEE Main

Question

A metal \(\mathrm{M}\) readily forms water soluble sulphate \(\mathrm{MSO}_{4}\), water insoluble hydroxide \(\mathrm{M}(\mathrm{OH})_{2}\) and oxide MO which becomes inert on heating. The hydroxide is soluble in \(\mathrm{NaOH}\). \(\mathrm{M}\) is (a) \(\mathrm{Mg}\) (b) \(\mathrm{Sr}\) (c) \(\mathrm{Be}\) (d) \(\mathrm{Ca}\)

Step-by-Step Solution

Answer

The metal M is Be (option c).

Step 1: Understanding the Metal Characteristics

We need to identify the metal "M" based on the given properties. It forms water-soluble sulfate (\(\text{MSO}_4\)), a water-insoluble hydroxide (\(\text{M(OH)}_2\)), and an oxide (\(\text{MO}\)) that becomes inert upon heating. The metal's hydroxide is soluble in \(\text{NaOH}\).

Step 2: Examine Water Solubility and Insolubility

The metal forms a sulfate \(\text{MSO}_4\) that is soluble in water, indicating it is likely an alkaline earth metal. The hydroxide \(\text{M(OH)}_2\) is insoluble in water, which suggests that \(\text{M}\) is \(\text{Be}\), \(\text{Mg}\), \(\text{Ca}\), or \(\text{Sr}\).

Step 3: Oxide Characteristics on Heating

The oxide \(\text{MO}\) becomes inert when heated. Metals like magnesium (\(\text{Mg}\)) form oxides that become stable upon heating, fitting this part of the description.

Step 4: Hydroxide Solubility in NaOH

The key property here is that the hydroxide \(\text{M(OH)}_2\) is soluble in \(\text{NaOH}\). Beryllium hydroxide \(\text{Be(OH)}_2\) is known for being soluble in \(\text{NaOH}\) due to its amphoteric nature, meaning it can dissolve in both acids and bases.

Step 5: Conclusion with Options

Based on the solubility of \(\text{MSO}_4\), the inert oxide \(\text{MO}\), and the solubility of \(\text{M(OH)}_2\) in \(\text{NaOH}\), the metal is \(\text{Be}\), since it is the only option where these characteristics are true. Thus, the answer is option (c) \(\text{Be}\).

Key Concepts

Solubility

Solubility refers to how well a substance can dissolve in a solvent, like water. In this context, the metal \( M \) we are discussing forms a sulfate compound, denoted as \( MSO_4 \), which is soluble in water. This means that when \( MSO_4 \) is added to water, it dissolves completely, creating a uniform solution.

In the case of alkaline earth metals, solubility varies across the group:

• For example, beryllium sulfate \( BeSO_4 \) is water-soluble, which aligns with the characteristics of metal "M" described in the exercise.
• Other sulfates such as calcium sulfate \( CaSO_4 \) have lower solubility in water.

Understanding the solubility of compounds helps in deducing which metal fits the criteria of having a soluble sulfate. This forms part of the basis for identifying beryllium (Be) as the metal in question.

Inert Oxides

An inert oxide is one that does not react easily with other substances, especially when heated. The oxide \( MO \) in our scenario becomes inert upon heating, meaning it remains stable and doesn't undergo any further reactions.

This property is particularly noteworthy among alkaline earth metals. For instance:

• Magnesium oxide \( MgO \) becomes stable when heated, not readily participating in chemical reactions.
• Similarly, beryllium oxide \( BeO \) is recognized for its stability at high temperatures, behaving inertly.

Recognizing that some oxides become inert upon heating aids in eliminating metals that don't match this behavior from the list of possibilities, further narrowing down the choice to beryllium.

Amphoteric Hydroxides

Amphoteric hydroxides possess both acidic and basic properties, allowing them to react with both acids and bases. In our case, the hydroxide \( M(OH)_2 \) is water-insoluble but dissolves in \( NaOH \), indicating its amphoteric nature.

Beryllium hydroxide \( Be(OH)_2 \) is a classic example of an amphoteric hydroxide:

• It doesn't dissolve in water, aligning with the properties of the metal listed in the exercise.
• Yet, it readily dissolves in NaOH due to its amphoteric quality, reacting with the base to form a complex compound.

This characteristic is crucial for identifying the metal "M" since among alkaline earth metals, beryllium uniquely exhibits this amphoteric behavior, thus confirming it as the metal in question.