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

Question

A white, water-insoluble solid \((\mathrm{A})\), 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 \(\mathrm{K}_{2} \mathrm{CrO}_{4}\) solution. Gas (B) bubbled through a suspension of \(\mathrm{CaCO}_{3}\) 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

The identity of compound (E) is \(\mathrm{CaCrO}_4\).

Step 1 - Identify the gas (B)

The gas (B) is described as colorless and odorless. Additionally, when bubbled through \(\mathrm{CaCO}_3\), it causes the dissolution of the compound. Carbon dioxide (\(\mathrm{CO}_2\)) is a colorless, odorless gas that reacts with calcium carbonate (\(\mathrm{CaCO}_3\)) to form calcium bicarbonate, which is soluble in water. Thus, gas (B) is \(\mathrm{CO}_2\).

Step 2 - Deduce the composition of solid (A)

Solid (A) reacts with an acid to release \(\mathrm{CO}_2\). A common water-insoluble carbonate that reacts with acids to release \(\mathrm{CO}_2\) is calcium carbonate (\(\mathrm{CaCO}_3\)). Therefore, solid (A) is likely \(\mathrm{CaCO}_3\).

Step 3 - Analyze Solution (C)

Upon adding an acid to \(\mathrm{CaCO}_3\), the clear solution (C) likely contains \(\mathrm{Ca}^{2+}\) ions due to the dissolution of \(\mathrm{CaCO}_3\).

Step 4 - Reactions with Solution (C)

Solution (C) gives a white precipitate (D) when \(\mathrm{H}_{2}\mathrm{SO}_{4}\) is added. The formation of a white precipitate indicates the formation of calcium sulfate \(\mathrm{CaSO}_4\), which is sparingly soluble, confirming the presence of \(\mathrm{Ca}^{2+}\) ions.

Step 5 - Reaction with Potassium Chromate

When \(\mathrm{K}_2\mathrm{CrO}_4\) is added to solution (C), a yellow precipitate forms. The yellow precipitate suggests the formation of a chromate compound containing calcium, such as calcium chromate (\(\mathrm{CaCrO}_4\)).

Key Concepts

Gas Identification

In the given exercise, identifying the colorless and odorless gas released when a solid reacts with acid is crucial. This gas, when bubbled through calcium carbonate offers an insightful clue. **Carbon dioxide** ( oinfo{CO} 2 ), a common gas in chemical reactions, fits this description perfectly. It is well-known for being colorless, odorless, and capable of reacting with calcium carbonate ( oCaCO 3 ) to form soluble calcium bicarbonate in water. When carbondioxide is involved, fizzing is often seen, representing gas release. This step should always be considered during gas identification, especially in reactions involving carbonates. This property is not only fundamental in labs wherever gases are released but also in everyday occurrences such as effervescent beverages.

Calcium Carbonate Reaction

The transformation of calcium carbonate ( oCaCO 3 ) when exposed to an acidic environment provides important insights into chemical reactions. Calcium carbonate, common in rocks and shells, is noted for its water-insolubility. Upon reacting with an acid, it transforms to release carbon dioxide gas, a hallmark reaction.

• This reaction also results in the formation of clear solutions, as seen with the generation of calcium ions ( Ca 2+ ).
• The resultant calcium ions become part of the solution as the carbonate compound dissolves.

This chemical property underscores the significance of categorizing compounds based on their reactions, especially for the identification of gas or ions in reaction mixtures.

Precipitation Reactions

A key feature of chemical analysis is the formation of precipitates. Upon the addition of certain reagents, a visible change such as precipitation verifies the presence of specific ions. For instance, in the original problem, the reaction between calcium ions and additional reagents leads to precipitates which help deduce the identities of mystery compounds: - Upon adding dilute sulphuric acid to the solution containing calcium ions, **calcium sulfate** ( CaSO 4 ) forms as a white precipitate. - Further, the addition of potassium chromate results in a **yellow precipitate** ( CaCrO 4 ), confirming the presence of chromate compounds. Precipitation reactions are a groundwork in inorganic chemistry. They are employed not just in identifying components in mixtures but also in various industrial processes.

Chemical Analysis Steps

Analyzing compounds by observing reactions and changes, like gas release and precipitate formation, follows methodical steps. Understanding how one step leads to another aids not only in identifying unknown substances but also in predicting reaction outcomes. In our exercise, the following steps illustrate thorough chemical analysis:

• Start with a known reaction, like the solid with acid - releasing gas.
• Move systematically to identify ions present by observing any precipitates formed upon further reactions.
• Confirm the presence of common ions like calcium by their expected reaction, such as precipitation by sulfate.

Employing these careful, deliberate steps ensures accurate identification of lesser-known compounds, which is pivotal in laboratories doing even simple chemical analysis.