Problem 136 - A Complete Resource Book in Chemistry for JEE Main

Question

The total number of diprotic acids among the following is [IIT 2010] \(\mathrm{H}_{3} \mathrm{PO}_{4} \quad \mathrm{H}_{2} \mathrm{SO}_{4} \quad \mathrm{H}_{3} \mathrm{PO}_{3} \quad \mathrm{H}_{2} \mathrm{CO}_{3} \quad \mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{7}\) \(\mathrm{H}_{3} \mathrm{BO}_{3} \quad \mathrm{H}_{3} \mathrm{PO}_{2} \quad \mathrm{H}_{2} \mathrm{CrO}_{4} \quad \mathrm{H}_{2} \mathrm{SO}_{3}\)

Step-by-Step Solution

Answer

There are 6 diprotic acids: \( ext{H}_{2} ext{SO}_{4}\), \( ext{H}_{3} ext{PO}_{3}\), \( ext{H}_{2} ext{CO}_{3}\), \( ext{H}_{2} ext{S}_{2} ext{O}_{7}\), \( ext{H}_{2} ext{CrO}_{4}\), \( ext{H}_{2} ext{SO}_{3}\).

Step 1: Understand diprotic acids

A diprotic acid is an acid that can donate two protons (hydrogen ions, H⁺) per molecule in the process of dissociation. This means it has two replaceable hydrogen atoms that can be released.

Step 2: Analyze each acid for dissociable hydrogen atoms

Each compound needs to be analyzed to determine how many hydrogen ions it can donate:1. \( ext{H}_{3} ext{PO}_{4}\) - Phosphoric acid, contains 3 hydrogens but only two are dissociable in aqueous medium; hence, it's triprotic overall. 2. \( ext{H}_{2} ext{SO}_{4}\) - Sulfuric acid, can donate two protons, making it diprotic.3. \( ext{H}_{3} ext{PO}_{3}\) - Only two hydrogens are dissociable; it's therefore diprotic.4. \( ext{H}_{2} ext{CO}_{3}\) - Carbonic acid, can donate two protons, so it's diprotic.5. \( ext{H}_{2} ext{S}_{2} ext{O}_{7}\) - Per-disulfuric acid, can donate two hydrogens.6. \( ext{H}_{3} ext{BO}_{3}\) - Boric acid, behaves atypically and is actually monoprotic.7. \( ext{H}_{3} ext{PO}_{2}\) - Hypophosphorous acid, is monoprotic because only one hydrogen is dissociable.8. \( ext{H}_{2} ext{CrO}_{4}\) - Chromic acid, can donate two protons, making it diprotic.9. \( ext{H}_{2} ext{SO}_{3}\) - Sulfurous acid, can donate two protons, making it diprotic.

Step 3: Count the diprotic acids

The diprotic acids in the given list are \( ext{H}_{2} ext{SO}_{4}\), \( ext{H}_{3} ext{PO}_{3}\), \( ext{H}_{2} ext{CO}_{3}\), \( ext{H}_{2} ext{S}_{2} ext{O}_{7}\), \( ext{H}_{2} ext{CrO}_{4}\), and \( ext{H}_{2} ext{SO}_{3}\). Count them to confirm the number of diprotic acids.

Key Concepts

Acid Dissociation

Acid dissociation is a fundamental concept in chemistry that describes how acids donate protons (H⁺ ions) when dissolved in water. A key aspect of acids is their ability to lose hydrogen ions, transforming into their conjugate base. For diprotic acids, the process is sequential, meaning two steps occur, each involving the release of one proton.
This concept is crucial in understanding how acids behave in solutions. Consider sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)), a diprotic acid. When it dissociates, the first step involves the release of one proton, resulting in the formation of the bicarbonate ion, and in the second step, another proton is released, yielding the sulfate ion.
It is important to recognize when an acid can donate more than one ion, as it affects the acidity and the pH of the solution. Knowing how many hydrogen ions an acid can release helps predict its behavior in chemical reactions.

Phosphoric Acid

Phosphoric acid, formula \(\mathrm{H}_{3} \mathrm{PO}_{4}\), can release three protons. However, in standard conditions, only two of these are likely to dissociate easily, classifying it as a diprotic acid in practice. Only in specific situations does the third proton dissociate.
The dissociation steps are crucial to understanding its behavior:

• In water, the first dissociation step releases one hydrogen ion, generating a dihydrogen phosphate ion.
• The second step loses another proton, producing a hydrogen phosphate ion.
• The third dissociation results in a phosphate ion when conditions favor complete dissociation, which is less common.

Phosphoric acid's dissociation makes it a versatile chemical, useful in fertilizers, food flavoring, and industrial applications.

Sulfuric Acid

Sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)) is iconic for being a strong diprotic acid, capable of releasing two protons. Its complete dissociation makes it a vital industrial chemical.
This acid dissociates in two distinct steps:

• The first hydrogen ion dissociates easily, forming the bisulfate ion.
• The second proton is released, forming the sulfate ion, defining its diprotic nature.

Due to its strong acidic properties, it is used in various applications like manufacturing fertilizers, refining oil, and as an electrolyte in lead-acid batteries. Its ability to donate two protons efficiently makes it particularly powerful.

Carbonic Acid

Carbonic acid (\(\mathrm{H}_{2} \mathrm{CO}_{3}\)) is a weak diprotic acid, often found in equilibrium with carbon dioxide in water. Its dissociation is a critical part of the carbon cycle and is key in natural processes.
Upon dissociation:

• The first step releases one proton, forming a bicarbonate ion.
• In the second step, another proton is released, forming a carbonate ion.

This gradual release influences buffering systems in biological and environmental contexts, such as blood pH and ocean acidity. Carbonic acid plays a significant role without being as strong as sulfuric acid, emphasizing its environmental importance.

Chromic Acid

Chromic acid (\(\mathrm{H}_{2} \mathrm{CrO}_{4}\)) is an interesting acidic compound, known for being diprotic. It can efficiently donate two protons.
The dissociation of chromic acid proceeds in two steps:

• First, a proton is lost, resulting in the hydrogen chromate ion.
• Second, losing another hydrogen ion, it forms the chromate ion.

Though primarily used in cleaning solutions and electroplating, handling chromic acid requires caution due to its high reactivity and potential health hazards. Its ability to donate two hydrogens places it among versatile and notable diprotic acids in industrial chemistry.

Sulfurous Acid

Sulfurous acid (\(\mathrm{H}_{2} \mathrm{SO}_{3}\)) is another example of a diprotic acid, although weaker than sulfuric acid. Its dissociation steps are similar in principle, but it is less complete than its sulfuring counterpart.
Steps include:

• The initial dissociation releases a proton, forming the bisulfite ion.
• The second proton's release results in the formation of the sulfite ion.

Sulfurous acid plays roles in preservative uses and as an intermediate in chemical synthesis. Despite its lower strength, its dissociative properties make it an important compound for study and industry.