Problem 11 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is a tribasic acid? (a) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) (c) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) (d) \(\mathrm{HPO}_{3}\)

Step-by-Step Solution

Answer

The tribasic acid is \( \mathrm{H}_{3} \mathrm{PO}_{4} \).

Step 1: Understand the Definition

A tribasic acid is an acid that can donate three hydrogen ions (protons) per molecule when dissolved in water. This means it has three ionizable hydrogen atoms.

Step 2: Analyze the Options

Examine the molecular formulas to determine the number of ionizable hydrogen atoms. - (a) \( \mathrm{H}_{3} \mathrm{PO}_{4} \) has three hydrogen atoms that can be potentially ionized.- (b) \( \mathrm{H}_{3} \mathrm{PO}_{3} \) actually has only two hydrogen atoms that can ionize, as it is diprotic.- (c) \( \mathrm{H}_{3} \mathrm{PO}_{2} \) has one hydrogen atom that can be ionized, making it monoprotic.- (d) \( \mathrm{HPO}_{3} \) cannot be tribasic as it does not have three hydrogens.

Step 3: Conclusion Based on Analysis

From the analysis, only \( \mathrm{H}_{3} \mathrm{PO}_{4} \) (phosphoric acid) can ionize three hydrogen atoms, making it the tribasic acid among the given options.

Key Concepts

Ionizable Hydrogen Atoms

In the context of acids, the term "ionizable hydrogen atoms" refers to the hydrogen atoms in a molecule that can be released as positive ions, or protons, when the acid is dissolved in water. These hydrogen atoms are critical because they determine the acid's ability to donate protons and thus its acidic nature.

When an acid molecule donates these ionizable hydrogen atoms, it undergoes a chemical reaction that releases them as \( ext{H}^+\) ions. This process occurs because the molecule dissociates, or breaks apart, into its constituent ions in an aqueous solution.

It's important to note that not all hydrogen atoms in a molecule are ionizable. Only those that are bonded in such a way that allows for ionization under normal conditions will dissociate to form \( ext{H}^+\) ions. For example:

• A tribasic acid, like phosphoric acid, can donate three protons.
• A dibasic acid can donate two protons.
• A monobasic acid can donate only one proton.

This concept is crucial for understanding how acids interact with other compounds and influence pH levels.

Phosphoric Acid

Phosphoric acid ( H_3PO_4 ) is a common example of a tribasic acid, which has the unique ability to donate three protons (hydrogen ions) due to its three ionizable hydrogen atoms. It's a crucial compound in various chemical processes and applications, ranging from fertilizers to food additives.

Being tribasic, phosphoric acid can undergo a three-stage ionization process:

• In the first ionization step, it loses one H^+ ion to form dihydrogen phosphate (H_2PO_4^-).
• In the second step, it loses a second H^+ ion to form hydrogen phosphate (HPO_4^{2-}).
• In the final step, it loses a third H^+ ion to produce phosphate ( PO_4^{3-}).

This sequential ionization makes phosphoric acid versatile since it can act at different strengths depending on the environmental conditions and requirements. The ability to donate three protons is what distinguishes it as a tribasic acid, vital in industrial and biological systems.

Acidic Protons

The term "acidic protons" describes the hydrogen ions that can be donated by an acid. These protons are what give an acid its "strength," with stronger acids able to donate protons more readily than weaker acids.

When phosphoric acid dissociates in water, it donates its acidic protons, contributing to the solution's acidity. Let's explore the basics of acidic protons:

• An acidic proton is simply a hydrogen atom that has lost its electron, resulting in a positively charged H^+ ion.
• The donation of acidic protons by an acid into solution is what increases the concentration of hydrogen ions in that solution. This increase leads to a decrease in pH.
• In practical terms, the more acidic protons an acid can donate, the stronger the effect it will have on the solution's pH.

Understanding acidic protons is essential for appreciating how acid-base reactions occur and why acids are integral to processes like digestion and chemical manufacturing.