Problem 38 - A Complete Resource Book in Chemistry for JEE Main

Question

Basicity of orthophosphoric acid is (a) 5 (b) 4 (c) 3 (d) 2

Step-by-Step Solution

Answer

The basicity of orthophosphoric acid is 3.

Step 1: Understanding Basicity

Basicity refers to the number of hydrogen ions (H⁺) that an acid can donate in a reaction. It's an important factor in determining the strength and behavior of acids in chemical reactions.

Step 2: Chemical Structure of Orthophosphoric Acid

Orthophosphoric acid is commonly known as phosphoric acid with the chemical formula H₃PO₄. It consists of three hydrogen atoms (H), one phosphorus atom (P), and four oxygen atoms (O).

Step 3: Determine the Number of Ionizable Hydrogen Atoms

In H₃PO₄, all three hydrogen atoms are capable of ionizing, meaning they can separate as hydrogen ions (H⁺) when dissolved in water.

Step 4: Conclusion on Basicity

Since orthophosphoric acid (H₃PO₄) has three ionizable hydrogen atoms, its basicity is 3.

Key Concepts

Orthophosphoric acid

Orthophosphoric acid, commonly known as phosphoric acid, is an important inorganic compound that is used in various industries, from agriculture to the food sector. Its chemical formula is H₃PO₄. This acid is known for its ability to donate hydrogen ions in a solution and plays a crucial role in several chemical reactions and processes. Understanding the fundamentals of orthophosphoric acid helps in predicting its behavior in different environments, particularly in aqueous solutions. It is characterized by its relatively high boiling point and viscous nature, making it a stable compound under standard conditions.

Ionizable hydrogen atoms

The concept of ionizable hydrogen atoms is central to understanding the behavior of acids like orthophosphoric acid. An ionizable hydrogen atom in a molecule can dissociate as a hydrogen ion (H⁺), which is crucial for the acid's function in a chemical reaction. For orthophosphoric acid, each of the three hydrogen atoms attached to the phosphate group can dissociate. When dissolved in water, these hydrogen atoms tend to separate, thereby increasing the hydrogen ion concentration in the solution. This process determines the basicity of the acid and is a key factor in evaluating its acidity and strength.

Acid strength

Acid strength refers to an acid's ability to donate hydrogen ions in solution. Strong acids dissociate completely in water, releasing more hydrogen ions, whereas weak acids do not dissociate fully. Orthophosphoric acid is classified as a weak acid because it does not completely ionize in water, meaning not all its hydrogen atoms are released as hydrogen ions.

• The greater the number of ionizable hydrogen atoms, the higher the potential for acidity.
• Despite having three potentially ionizable hydrogen atoms, orthophosphoric acid is only weakly acidic.
• This partial ionization affects its role and efficiency in reactions involving proton transfer.

Understanding this behavior helps predict how the acid will interact in various chemical equations and reactions.

Phosphoric acid chemical structure

The chemical structure of phosphoric acid is crucial to understanding its properties and reactivity. Phosphoric acid has the molecular formula H₃PO₄, consisting of:

• Three hydrogen (H) atoms
• One phosphorus (P) atom
• Four oxygen (O) atoms

This structure forms a tetrahedral shape where the phosphorus atom is centrally located, bonded to four oxygen atoms. One of these oxygen atoms is double bonded to phosphorus, while the others are single bonded, each with a hydrogen ion. This arrangement allows the hydrogen atoms to ionize when the acid is in solution, an action central to its function as an acid. The ability of these hydrogen atoms to ionize is what gives orthophosphoric acid its basicity of 3, as all three hydrogen atoms can dissociate and therefore potentially be involved in chemical reactions.