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 (d) basic lead carbonate (c) strontium carbonate

Step-by-Step Solution

Answer

The metal salt is most likely barium carbonate.

Step 1: Analyze the Reaction with Potassium Chromate

When a metallic salt forms a yellow precipitate with potassium chromate, it suggests the presence of lead or barium ions. Both Pb and Ba chromates are yellow.

Step 2: Consider the Precipitate with Dilute Sulfuric Acid

The reaction with dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\) forms a white precipitate, which indicates the formation of a sulfate. Strontium and barium sulfates are white and insoluble. Lead sulfate is also white.

Step 3: Examine Reactions with Sodium Chloride and Iodide

Lead forms precipitates with iodine (yellow) and chloride (white). If the salt does not give precipitates with sodium chloride or iodide, it eliminates lead options automatically. Barium and strontium do not form precipitates with these.

Step 4: Identify the Likely Candidate

Given these characteristics, we can narrow the options. Since lead was eliminated due to non-precipitation with chloride and iodide, and a sulfate was formed, the metal shares properties with strontium. Barium carbonate is a logical candidate since strontium acetate would react similarly to barium, but is not typically yellow.

Key Concepts

Metal Salt Reactions

Metal salt reactions are fundamental concepts in inorganic chemistry, where metals react with salts to produce various outcomes like precipitates. These reactions can provide clues about the composition and properties of the involved substances. For instance, the reaction of a metal salt with potassium chromate can result in a colorful precipitate. This is because metal ions, such as lead (Pb) or barium (Ba), form insoluble chromate compounds. These reactions are easy to observe due to their striking colors and assist in identifying the metal ions present in a solution.

• Metal ions combine with chromates to form chromate salts.
• Color changes can indicate the presence of specific metal ions.
• Lead and barium are known to form yellow precipitates with chromate.

Through such reactions, chemists can deduce which metal ions are present in a solution and apply suitable chemical processes to separate or identify these ions.

Chromate Precipitation

Chromate precipitation involves the formation of a solid from a solution when chromate ions react with certain metal cations. The result is often a bright, distinct color due to the insolubility of metal chromates. When a solution containing lead ions (Pb²⁺) is treated with potassium chromate (K₂CrO₄), a yellow precipitate of lead chromate (PbCrO₄) forms. This property is exploited in chemical analysis to test for the presence of lead or barium.

• Chromate ions react with specific metal ions to form insoluble salts.
• The color of the precipitate aids in identifying the metal ions.
• Lead and barium chromates are distinctive for their bright yellow hue.

These reactions are important in analytical chemistry because they allow for the easy identification of metals in a mixture based on the color and formation of the precipitate.

Sulfate Formation

Sulfate formation can be observed when sulfate compounds react with metal ions, forming a precipitate that is often white and insoluble. The reaction of metal ions with dilute sulfuric acid (\( \mathrm{H}_{2}\mathrm{SO}_{4} \)) can illustrate the presence of sulfate-forming metals. For example, when strontium (Sr²⁺) or barium (Ba²⁺) ions are reacted with sulfate ions, they form strontium sulfate (SrSO₄) or barium sulfate (BaSO₄) respectively. Both are insoluble in water, resulting in a white precipitate.

• Sulfate ions combine with metal ions to form sulfates that are typically white.
• This method distinguishes ions by their solubility characteristics.
• Barium and strontium sulfates are examples of insoluble white sulfates.

By understanding sulfate formation, chemists can identify which metal ions are present based on their reactions with sulfuric acid.

Ionic Compounds

Ionic compounds are composed of positive and negative ions held together by strong electrostatic forces. These compounds typically form between metals and non-metals or between ions with opposite charges. In the context of reactions with metal salts, the ionic nature dictates the solubility and reaction products observed. For instance, when metal salts are mixed with solutions like sodium chloride or iodide, if no precipitate forms, it implies that the metal ions present do not react to form insoluble ionic compounds in those particular reactions.

• Ionic compounds are formed from cations and anions.
• The nature of ions determines the solubility and precipitate formation.
• Barium and strontium ion reactions do not usually result in precipitates with sodium chloride or iodide.

Understanding the characteristics of ionic compounds is essential in predicting the outcome of chemical reactions and in identifying the presence of specific ions in a solution.