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 in the context of acids refers to the number of hydrogen ions (H⁺) the acid can donate in a solution. It corresponds to the number of replaceable hydrogen ions in one molecule of the acid.

Step 2: Identify the Acid

Orthophosphoric acid is chemically written as H₃PO₄. It is a weak acid and dissociates in steps to donate hydrogen ions.

Step 3: Counting Replaceable Hydrogen Atoms

For H₃PO₄, there are three hydrogen atoms that can potentially be ionized and replaced by basic radicals or can be donated as H⁺ ions. Each step of dissociation involves the release of one H⁺ ion. Thus, orthophosphoric acid can release three H⁺ ions.

Step 4: Determining Basicity

The basicity of an acid is the total number of H⁺ ions it can donate. Since orthophosphoric acid (H₃PO₄) can release three hydrogen ions, its basicity is 3.

Step 5: Choosing the Correct Option

From the provided options, the correct choice is (c) 3, as H₃PO₄ has three replaceable hydrogen ions, determining its basicity as 3.

Key Concepts

Orthophosphoric Acid

Orthophosphoric acid, also known as phosphoric acid, is a commonly encountered substance in chemistry. Its chemical formula is \( \text{H}_3\text{PO}_4 \). This formula indicates the presence of three hydrogen atoms, one phosphorus atom, and four oxygen atoms.

In practice, orthophosphoric acid serves as a weak acid. This means it does not fully dissociate in solution, unlike strong acids like hydrochloric acid (HCl). Instead, it partially ionizes, making it a prime example in studies about hydrogen ion donations and acid dissociation.

With its role in the food industry as a preservative and its use in fertilizers, understanding the characteristics of orthophosphoric acid is not only helpful but essential to several fields. It's a versatile compound with vital applications, and knowing its properties allows us to utilize it effectively.

Hydrogen Ions Donation

Hydrogen ions (H⁺) donation is a key concept in understanding acids. When acids like orthophosphoric acid dissolve in water, they release hydrogen ions into the solution. This process not only determines the acidity of the solution but also explains the acid's basicity, or the number of replaceable or ionizable hydrogen ions.

For orthophosphoric acid, this involves donating up to three hydrogen ions. It's important to remember that with each ionization step, one hydrogen ion is released, contributing to the solution's acidity.

• First step: Releases one H⁺ ion.
• Second step: Releases another H⁺ ion.
• Third step: Releases the final H⁺ ion.

Each step results in a different degree of ionization, affecting the acid's concentration of H⁺ ions in the solution and thereby altering its pH level. Understanding this helps to predict reactions and behavior when orthophosphoric acid interacts with other chemical species.

Acid Dissociation Steps

The dissociation of orthophosphoric acid occurs in distinct steps, with each step involving the gradual loss of a hydrogen ion. These steps reflect its behavior as a triprotic acid, meaning it can undergo three dissociation processes.

Here's a breakdown of the dissociation steps in orthophosphoric acid:

• First dissociation: \( \text{H}_3\text{PO}_4 \rightarrow \text{H}^+ + \text{H}_2\text{PO}_4^- \)
• Second dissociation: \( \text{H}_2\text{PO}_4^- \rightarrow \text{H}^+ + \text{HPO}_4^{2-} \)
• Third dissociation: \( \text{HPO}_4^{2-} \rightarrow \text{H}^+ + \text{PO}_4^{3-} \)

Each step signifies the release of one hydrogen ion and thus incrementally increases the solution's acidity. This incremental release is why the basicity of orthophosphoric acid is three, equaling the number of potential hydrogen ions the molecule can donate. This process models the step-wise nature of acid reactions in aqueous solutions, underlying many principles in chemistry.