Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The normality of \(0.3 \mathrm{M}\) phosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{3}\right)\) is, [1999 - 2 Marks] (a) \(0.1\) (b) \(0.9\) (c) \(0.3\) (d) \(0.6\)

Step-by-Step Solution

Answer

The normality is 0.6, which is option (d).

Step 1: Understanding Normality

Normality is a measure of concentration equivalent to molarity multiplied by the number of equivalents per mole of solute. In acid-base chemistry, it is often used to represent the equivalence of acid or base in terms of protons (H+).

Step 2: Determining Equivalents for Phosphorous Acid

Phosphorous acid \(\mathrm{H}_{3}\mathrm{PO}_{3}\) can donate 2 protons (H+), despite having three hydrogens. This is because one hydrogen is not ionizable as it is directly bonded to the phosphorus atom. Thus, its number of equivalents is 2.

Step 3: Calculating Normality

The normality (N) is calculated using the formula: \(\text{Normality} = \text{Molarity} \times \text{Equivalents}\text{ per mole}\). For \(\mathrm{H}_{3}\mathrm{PO}_{3}\), \(N = 0.3 \, \text{M} \times 2 = 0.6\).

Step 4: Selecting the Correct Option

From the options given, the calculated normality, 0.6, corresponds to option (d).

Key Concepts

Acid-Base Chemistry

Acid-base chemistry is an essential part of chemistry that focuses on acids, bases, and their interactions in solutions. In this field, we explore how acids and bases react, how they can neutralize each other, and how their concentration is measured. Key concepts in acid-base chemistry include:

• Acids: Substances that can donate protons ( H^+ ) in a solution. They can also accept an electron pair in reactions.
• Bases: Substances capable of accepting a proton or donating an electron pair to create a chemical bond.
• Neutralization: This is a reaction where an acid and a base react to form water and a salt, effectively neutralizing the acid and base.

A foundational concept in acid-base chemistry is the idea of concentration, which is often measured in terms of molarity (moles per liter) or normality. Normality is particularly useful because it takes into account the equivalence factor of the acid or base participating in the reaction, an important feature for precise calculations in titrations and reactions.

Phosphorous Acid

Phosphorous acid H_3PO_3 , is a technical, but interesting compound in acid-base chemistry. Although it possesses three hydrogen atoms, only two of them are ionizable, meaning they can donate protons. This peculiarity stems from its structure:

• Two of the hydrogen atoms are bonded to oxygen atoms, and these are the ones that can dissociate as protons ( H^+ ).
• The third hydrogen atom is directly bonded to the phosphorus atom, making it non-ionizable under normal conditions, as it does not participate in the typical acid dissociation.

This means that phosphorous acid is partially protic. Due to this characteristic, even though it might seem at first glance that it should behave like a triprotic acid, in reality, it only behaves as a diprotic acid due to the two protons it can release.

Equivalents per Mole

To fully grasp normality in the context of acid-base reactions, understanding the concept of "equivalents per mole" is crucial. This concept measures how many moles of reactive capacity (protons or hydroxide ions) a substance can provide or accept in a reaction. For acids and bases, this can be understood as:

• In acids, the equivalent is the number of protons that can be potentially donated. In H_3PO_3 the number is 2, accounting for its 2 ionizable hydrogens
• In bases, it is how many hydroxide ions a base can donate or how many protons a base can accept.

To calculate normality, one needs to multiply the molarity (the concentration of the solution in moles per liter) by the number of equivalents per mole. This calculation forms a bridge between the conceptual understanding of acid-base reactions and their practical computation in chemical equations. Understanding equivalents per mole helps chemists correctly figure out how much of a substance is truly reactive in a solution, leading to accurate calculations and predictions in chemical processes.