Problem 86 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following observation \(\mathrm{M}^{\mathrm{n}+}+\mathrm{HCl} \longrightarrow\) white precipitate Water soluble The metal ion \(\mathrm{M}^{\mathrm{n}+}\) will be (a) \(\mathrm{Pb}^{2+}\) (b) \(\mathrm{Hg}^{2+}\) (c) \(\mathrm{Sn}^{2+}\) (d) \(\mathrm{Ag}^{+}\)

Step-by-Step Solution

Answer

The metal ion is \( \mathrm{Sn}^{2+} \).

Step 1: Identify the Reaction with HCl

The observation shows that a metal ion \( M^{n+} \) reacts with \( \text{HCl} \) to form a white precipitate. This indicates that \( M^{n+} \) forms an insoluble chloride.

Step 2: Evaluate Water Solubility of the Precipitate

The precipitate formed is water-soluble. This means that while an initial precipitation occurs, the compound should dissolve upon addition of water.

Step 3: Analyze Given Options

Evaluate each option for its ability to form a white precipitate that is also water-soluble:- \( \mathrm{Pb}^{2+} \) forms \( \text{PbCl}_2 \), a white precipitate that is only slightly soluble in water.- \( \mathrm{Hg}^{2+} \) forms \( \text{HgCl}_2 \), which does not form such a precipitate.- \( \mathrm{Sn}^{2+} \) forms \( \text{SnCl}_2 \), a white precipitate that is soluble in hot water.- \( \mathrm{Ag}^{+} \) forms \( \text{AgCl} \), a white precipitate that is insoluble in water.

Step 4: Conclusion on Soluble Precipitate

\( \text{SnCl}_2 \), formed by \( \mathrm{Sn}^{2+} \), is the only option that becomes soluble under certain conditions (in hot water), matching the description of a water-soluble precipitate.

Key Concepts

Solubility

When a substance dissolves in a liquid, it is said to be soluble. The concept of solubility refers to the maximum amount of a substance that can dissolve in a certain volume of solvent at a specified temperature. Solubility is crucial in understanding chemical reactions because it often dictates if substances can mix at the molecular level and react.

Some key points related to solubility include:

• Temperature: Generally, the solubility of solids in liquids increases with temperature. This means more solid can dissolve in warmer liquids compared to cooler ones.
• Saturation: A solution is saturated when it contains the maximum amount of solute that can dissolve at a particular temperature.
• Water Solubility: When a precipitate dissolves in water upon heating, like with \( ext{SnCl}_2\), it indicates a higher solubility in hot water compared to cold.

Understanding solubility helps in predicting how compounds will behave when mixed with solutions like \( ext{HCl}\) in the initial exercise.

Precipitation

Precipitation refers to the formation of a solid in a solution during a chemical reaction. When two solutions are mixed, and an insoluble product is formed, it separates out of the solution as a precipitate. Precipitates are often formed when an ionic compound becomes insoluble in the reaction solvent.

Here are some highlights:

• Appearance: Precipitates typically appear as cloudy suspensions in the solution and settle over time.
• Solubility Influence: Even if a precipitate forms, it can sometimes dissolve if conditions change, such as an increase in temperature.
• Insoluble Chlorides: Chlorides like \( ext{AgCl}\) are classic examples of white precipitates that are notably insoluble in water.

The precipitation process helps identify different metal ions based on their chloride's solubility, as was analyzed in the solution steps of the exercise.

Metal Chlorides

Metal chlorides are ionic compounds made from metal ions and chloride ions. They are central to many chemical reactions, particularly in identifying different metal ions, as they can form distinctive compounds.

Important points about metal chlorides include:

• Formation: Metal chlorides are formed when a metal ion reacts with hydrochloric acid .
• Solubility and Precipitation: Some metal chlorides, such as \( ext{PbCl}_2\) and \( ext{AgCl}\), are insoluble, forming precipitates. Others, like \( ext{SnCl}_2\), transition from an insoluble to a soluble form under certain conditions, such as increased temperature.
• Identification of Metals: Different metal chlorides have unique solubility profiles, making them useful for distinguishing between metal ions based on their chloride's behavior in reactions.

Understanding how these compounds react helps in selecting the correct metal ion, such as \( ext{Sn}^{2+}\), in our original exercise scenario.