Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

A metal salt solution forms a yellow precipitate with potassium chromate in acetic acid, a white precipitate with dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\) but gives no precipitate with sodium chloride or iodide, it is (a) barium carbonate (b) lead carbonate (c) strontium carbonate (d) basic lead carbonate

Step-by-Step Solution

Answer

The correct answer is (d) basic lead carbonate.

Step 1: Analyze Yellow Precipitate Formation

The metal salt forms a yellow precipitate with potassium chromate ( K2CrO4 ). This reaction is characteristic of the presence of lead or barium salts, as both can form chromates. However, the yellow precipitate typically suggests the presence of lead chromate, PbCrO4 , which is distinctively yellow.

Step 2: Analyze White Precipitate with Dilute H2SO4

The metal salt forms a white precipitate with dilute sulfuric acid ( H2SO4 ). Both barium and lead can form white precipitates with sulfuric acid, corresponding to BaSO4 for barium and PbSO4 for lead.

Step 3: Analyze Non-Precipitation with Sodium and Iodide

The solution does not form precipitates with sodium chloride ( NaCl ) or sodium iodide ( NaI ). Barium does not form insoluble chlorides or iodides, but lead(II) usually forms insoluble white lead(II) chloride ( PbCl2 ) and yellow lead(II) iodide ( PbI2 ). Thus, the presence of lead in a non-reactive form must be considered.

Step 4: Evaluation of Options

Reviewing the given options: (a) and (c) describe barium and strontium carbonate, respectively, which do not entirely match the reactivity described. Both forms of lead carbonate, options (b) and (d), align more closely with the precipitate behaviors observed, particularly since (b) does not mention reactivity against common halides while (d), as basic lead carbonate, generally aligns with these results.

Key Concepts

Precipitate Formation

In chemistry, a precipitate is a solid that forms out of a liquid solution during a chemical reaction. When certain conditions are met, such as a chemical reaction or a change in temperature or concentration, the ions in the liquid solution come together to form a solid, which then falls out of the solution. This process is called precipitation.

Precipitation is often used in qualitative analysis to identify substances in a solution. Different substances will form distinct colored precipitates, which can help in recognizing them. For example, when a metal salt reacts with potassium chromate, a yellow precipitate is often indicative of lead chromate formation.

Precipitation reactions can be influenced by factors such as temperature, concentration of reactants, and the presence of other ions in the solution. Lower solubility in the solution leads to precipitate formation, as insoluble compounds tend to separate from the liquid phase.

Metal Salt Reactions

Metal salts are formed when metals react with acids forming ionic compounds. These reactions are characterized by the neutralization of the metal and the acid. When metal salts dissolve in water, they dissociate into metal cations and salt anions.

Reactions involving metal salts are fundamental in predicting and determining the behavior of specific metals in various environments. In the case of lead salts, different anions lead to different reactions and product formation. For instance, when lead salt reacts with chromates, sulfates, or halides, we see variations in precipitate color and solubility, which helps in identifying the metal involved.

Understanding these reactions is crucial in many fields such as laboratory analysis, environmental science, and industrial applications. The behavior of metal salts under different conditions helps chemists decipher their composition and reactivity in mixtures.

Chromate Test

The chromate test is an important analytical test used to identify the presence of specific metal ions in a solution. Potassium chromate serves as a reagent in these tests. When combined with a metal salt, if a chromate salt forms, the resultant precipitate's color can indicate the metal present.

Lead salts, for instance, typically form a distinctive yellow precipitate known as lead chromate when reacting with potassium chromate. This is a key test to pinpoint lead among different metal ions.

The chromate test is specifically useful due to its ability to produce highly visible precipitates, making it easier to conduct qualitative analysis even with minimal quantities present. However, care must be taken as chromates are toxic, requiring proper laboratory handling and disposal procedures.

Sulfuric Acid Test

The sulfuric acid test is another method used in chemical analysis to test for metal ions. By adding dilute sulfuric acid to a metal salt solution, you can observe whether a white precipitate forms. This precipitate often suggests the presence of certain metal sulfates which are relatively insoluble in water.

Barium sulfate and lead sulfate are two common examples of white precipitates formed with sulfuric acid. Both of these sulfates form when sulfuric acid interacts with solutions containing barium or lead ions, respectively.

This test is a cornerstone in qualitative analysis, aiding in the identification of metals based on the solubility of their sulfate forms. It's primarily used because of its simplicity and the distinct colorless precipitates it forms, making it functional in rapid testing and environmental monitoring.