Problem 257 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

There are 5 diprotic acids.

Step 1: Define Diprotic Acid

A diprotic acid is an acid that can donate two protons (hydrogen ions, \( H^+ \)) per molecule in an aqueous solution.

Step 2: Analyze Given Compounds

List and analyze each compound to determine how many protons can be donated.1. \( \mathrm{H}_3 \mathrm{PO}_4 \), phosphoric acid can donate 3 protons, so it's triprotic.2. \( \mathrm{H}_2 \mathrm{SO}_4 \), sulfuric acid can donate 2 protons, so it's diprotic.3. \( \mathrm{H}_3 \mathrm{PO}_3 \), phosphorous acid can donate 2 protons, so it's diprotic.4. \( \mathrm{H}_2 \mathrm{CO}_3 \), carbonic acid can donate 2 protons, so it's diprotic.5. \( \mathrm{H}_2 \mathrm{~S}_2 \mathrm{O}_7 \), pyrosulfuric acid can donate 2 protons, so it's diprotic.6. \( \mathrm{H}_3 \mathrm{BO}_3 \), boric acid typically acts as a monoprotic acid.7. \( \mathrm{H}_3 \mathrm{PO}_2 \), hypophosphorous acid can donate 1 proton, so it's monoprotic.8. \( \mathrm{H}_2 \mathrm{CrO}_4 \), chromic acid can donate 2 protons, so it's diprotic.

Step 3: Count Diprotic Acids

From the analysis: \( \mathrm{H}_2 \mathrm{SO}_4 \), \( \mathrm{H}_3 \mathrm{PO}_3 \), \( \mathrm{H}_2 \mathrm{CO}_3 \), \( \mathrm{H}_2 \mathrm{~S}_2 \mathrm{O}_7 \), and \( \mathrm{H}_2 \mathrm{CrO}_4 \) are diprotic acids. Count them.

Key Concepts

Donate Protons

In the world of chemistry, protons are positively charged particles, denoted by the symbol \( H^+ \). When we refer to acids, these substances have the unique ability to donate protons to other substances, typically in a dissolving medium like water. This process is known as acid dissociation.

A diprotic acid, as the name suggests, is capable of donating two protons per molecule. This is an important feature because each donation step can lead to sequential reactions, such as the ionization of sulfuric acid \( \text{H}_2\text{SO}_4 \): first to form bisulfate \( \text{HSO}_4^- \), and then completely to sulfate \( \text{SO}_4^{2-} \). This process is fundamental in many chemical reactions, including those that occur in the water around us and in industrial applications.

• Diprotic acids undergo two stages of proton donation.
• They play a critical role in buffering solutions and maintaining pH stability.
• Each dissociation step has a unique equilibrium constant.

Aqueous Solution

An aqueous solution is a homogenous mixture where a substance is dissolved in water. This is one of the most common forms in which chemicals interact and react.

In an aqueous solution, water acts as the solvent. It has the ability to dissolve a wide range of substances, earning it the nickname "universal solvent." When acids dissolve in water, they release protons into the solution, which is crucial for determining the acidity (or pH) of the solution.

• Acids in water release protons, increasing the \( H^+ \) concentration.
• This \( H^+ \) concentration is what gives the solution its acidic properties.
• The presence of these protons can lead to various chemical reactions.

Acid Classification

Acids are classified based on their ability to donate protons. For this reason, understanding the strength and type of an acid is crucial in predicting its behavior in reactions.

There are several classes of acids:

• Monoprotic Acids: Can donate one proton per molecule, like acetic acid (\( \text{CH}_3\text{COOH} \)).
• Diprotic Acids: As mentioned earlier, capable of donating two protons, like carbonic acid (\( \text{H}_2\text{CO}_3 \)).
• Triprotic Acids: Can donate three protons, as seen in phosphoric acid (\( \text{H}_3\text{PO}_4 \)).

The classification depends greatly on the molecule's chemical structure and the strength of the bonds holding the protons. Understanding this classification helps in anticipating how an acid will behave under certain chemical environments, and what kind of reactions it will undergo. Each step of donation in polyprotic acids has its own significance and affects the overall reaction dynamics.