Problem 120 - A Complete Resource Book in Chemistry for JEE Main

Question

A white, water-insoluble solid (A), is attacked by an acid, liberating a colourless, odourless gas (B), leaving a clear solution (C). The solution (C) gives a white precipitate (D) on addition of dilute \(\mathrm{H}_{2} \mathrm{SO}_{4},(\mathrm{C})\) gives a yellow precipitate (E) on addition of \(\mathrm{K}_{2} \mathrm{Cr} \mathrm{O}_{4}\) solution. Gas (B) bubbled through a suspension of \(\overrightarrow{\mathrm{CaCO}}_{3}\) causesdis solution of that compound. Identify (E). (a) \(\mathrm{CaCrO}_{4}\) (b) \(\mathrm{BaCr}_{2} \mathrm{O}_{7}\) (c) \(\mathrm{BaCrO}_{4}\) (d) none of these

Step-by-Step Solution

Answer

(E) is \(\mathrm{BaCrO}_{4}\), option (c).

Step 1: Identify the Gas (B)

Given that gas (B) is colorless and odorless and causes the dissolution of \( \overrightarrow{\mathrm{CaCO}}_{3} \), we can identify it as \( \mathrm{CO}_{2} \). Carbon dioxide bubbles through calcium carbonate, resulting in the formation of calcium hydrogen carbonate, which is soluble in water.

Step 2: Understand the Reaction of Solid (A)

Solid (A) reacts with an acid to produce \( \mathrm{CO}_{2} \), suggesting that (A) is likely a carbonate. Specifically, it could be \( \mathrm{BaCO}_{3} \) since the clear solution (C) afterwards results in specific precipitate reactions typical of barium compounds.

Step 3: Precipitate Formation with (C)

The solution (C) gives a white precipitate (D) when treated with dilute \( \mathrm{H}_{2}\mathrm{SO}_{4} \), likely indicating the formation of \( \mathrm{BaSO}_{4} \). This confirms the presence of barium in the solution.

Step 4: Reaction with \( \mathrm{K}_{2}\mathrm{CrO}_{4} \)

Solution (C) gives a yellow precipitate (E) when treated with \( \mathrm{K}_{2} \mathrm{CrO}_{4} \). Barium ions (\( \mathrm{Ba}^{2+} \)) react with \( \mathrm{K}_{2} \mathrm{CrO}_{4} \) to form \( \mathrm{BaCrO}_{4} \), which is a yellow precipitate.

Step 5: Identify Precipitate (E)

From the reaction with \( \mathrm{K}_{2} \mathrm{CrO}_{4} \), the yellow precipitate (E) is \( \mathrm{BaCrO}_{4} \), matching option (c).

Key Concepts

Carbonates

Carbonates are fascinating compounds often found in minerals and rocks. They contain the carbonate ion, written as \( \mathrm{CO}_3^{2-} \), which features a carbon atom surrounded by three oxygen atoms in a trigonal planar arrangement. Carbonates are commonly used in various industries due to their versatility. A key chemical property of carbonates is their reaction with acids, which releases carbon dioxide \( \mathrm{CO}_2 \) gas. This reaction is represented as follows:
\[ \mathrm{CaCO}_3 + 2 \mathrm{HCl} \rightarrow \mathrm{CaCl}_2 + \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \]
\( \mathrm{CO}_2 \) is the colorless, odorless gas released in such hydrolysis reactions.

• In the provided exercise, the formation of \( \mathrm{CO}_2 \) gas identified solid (A) as a carbonate, most probably a barium carbonate \( \mathrm{BaCO}_3 \).
• Carbonates like \( \mathrm{CaCO}_3 \) react with acids and help identify gases formed in chemical reactions, such as in the described precipitation reaction.

Barium Compounds

Barium compounds are known for their distinctive properties and are widely used in analytical chemistry. These include barium nitrate, barium sulfate, and barium chromate among others. In the context of the original exercise, barium carbonate \( \mathrm{BaCO}_3 \) is particularly relevant as it hints towards the identity of solid (A).
One feature of barium compounds is their ability to form insoluble precipitates in solution. This makes them useful for understanding chemical reactions in a test tube setting.

• For example, mixing a barium compound like \( \mathrm{BaCl}_2 \) with \( \mathrm{H}_2\mathrm{SO}_4 \) results in the formation of \( \mathrm{BaSO}_4 \), an insoluble white precipitate.
• This property allows for identification of barium in solutions, as was necessary in the step-by-step solution where the white precipitate \( \mathrm{BaSO}_4 \) formed.
• The formation of \( \mathrm{BaCrO}_4 \), a yellow precipitate when treated with \( \mathrm{K}_2\mathrm{CrO}_4 \), helps confirm the presence of barium ions in a solution.

Precipitation Reaction

Precipitation reactions are processes where two solutions mix, resulting in the formation of an insoluble solid known as a precipitate. This type of chemical reaction can be used to test for the presence of specific ions in a solution. They follow predictable patterns based on solubility rules.
In the context of the exercise presented, there are two main precipitation reactions:

• First, when solution \( \mathrm{C} \), containing dissolved barium ions, reacted with \( \mathrm{H}_2\mathrm{SO}_4 \). It produced a white precipitate of \( \mathrm{BaSO}_4 \), highlighting the formation of an insoluble sulfate salt.
• Secondly, the yellow precipitate \( \mathrm{BaCrO}_4 \) formed with \( \mathrm{K}_2\mathrm{CrO}_4 \) indicates the presence of barium ions, utilizing the specific reaction of these chemicals to create barium chromate.

These reactions are powerful tools in analytical chemistry to confirm the presence of particular ions and compounds in a mixture.