Problem 125 - A Complete Resource Book in Chemistry for JEE Main

Question

The equivalent mass of phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\) in the reaction, \(\mathrm{NaOH}+\mathrm{H}_{3} \mathrm{PO}_{4} \longrightarrow \mathrm{NaH}_{2} \mathrm{PO}_{4}+\mathrm{H}_{2} \mathrm{O}\) is (a) 98 (b) 89 (c) 49 (d) 58

Step-by-Step Solution

Answer

The equivalent mass of \(\mathrm{H}_3\mathrm{PO}_4\) is 98 g/equivalent.

Step 1: Determine Molar Mass of \(\mathrm{H}_3 \mathrm{PO}_4\)

Calculate the molar mass of \(\mathrm{H}_3 \mathrm{PO}_4\) by adding the atomic masses of its constituent elements: three hydrogen atoms, one phosphorus atom, and four oxygen atoms. Using atomic masses, we have \(3 \times 1 + 31 + 4 \times 16 = 98 \, \text{grams/mole}.\) So, the molar mass of \(\mathrm{H}_3 \mathrm{PO}_4\) is 98 g/mol.

Step 2: Identify the Reaction Type

In the reaction provided, \(\mathrm{NaOH}\) reacts with \(\mathrm{H}_3 \mathrm{PO}_4\) to form \(\mathrm{NaH}_2 \mathrm{PO}_4\) and \(\mathrm{H}_2 \mathrm{O}\). This is an acid-base neutralization reaction where \(\mathrm{H}_3 \mathrm{PO}_4\) donates one proton (\(\mathrm{H}^+\)).

Step 3: Determine Equivalent Mass

The equivalent mass of an acid is calculated as the molar mass divided by the number of replaceable hydrogen ions (protons) per molecule. Since \(\mathrm{H}_3 \mathrm{PO}_4\) donates one proton in this reaction, its equivalent mass is \(\frac{98}{1} = 98\, \text{grams/equivalent}.\)

Key Concepts

Molar Mass Calculation

To understand molar mass calculation, let's first think of molar mass as the weight of one mole of a substance. It's like finding the total mass of a box of chocolates where each piece has a different weight. For phosphoric acid, \( \mathrm{H}_{3}\mathrm{PO}_{4} \), we add up the atomic masses of all the atoms present in one molecule:

• Hydrogen: 3 atoms, each with a mass of about 1 gram/mole.
• Phosphorus: 1 atom with a mass of about 31 grams/mole.
• Oxygen: 4 atoms, each with a mass of about 16 grams/mole.

Adding these up gives us \(3 \times 1 + 31 + 4 \times 16 = 98 \) grams/mole. This sum is the molar mass of phosphoric acid and is helpful to calculate amounts in chemical reactions efficiently.

Acid-Base Neutralization

Acid-base neutralization is a fundamental concept where an acid reacts with a base to form water and a salt. Imagine a seesaw balancing act where one side gives and one side takes until it evens out. In our exercise, \( \mathrm{NaOH} \), a base, reacts with \( \mathrm{H}_{3}\mathrm{PO}_{4} \), an acid, to yield \( \mathrm{NaH}_{2}\mathrm{PO}_{4} \) and water.
This forms the foundation of neutralization reactions.
It typically involves:

• A transfer of protons from acid to base.
• The production of water, which is neither acidic nor basic.

Neutralizations are so important because they are common both in laboratories and in everyday applications. Understanding these reactions helps you predict the products formed.

Proton Donation

In the context of acid-base chemistry, proton donation refers to the process where an acid provides protons (\( \mathrm{H}^+ \)) to a base. Think of the acid as a friendly neighbor giving away apples, which in this case are protons. \( \mathrm{H}_{3}\mathrm{PO}_{4} \) donates a proton to \( \mathrm{NaOH} \).
This single proton donation is key to determining equivalent mass and reaction outcome.
Key points about proton donation include:

• Acids are defined by their ability to donate protons.
• The number of protons an acid can donate influences reactions and equivalence.

This concept not only helps to identify acids and bases but also to calculate equivalent masses and predict reaction results.

Chemical Reactions

Chemical reactions are processes where substances, the reactants, are transformed into different substances, the products. It’s like following a recipe where each ingredient mixed a certain way results in a new dish. In our given reaction, \( \mathrm{NaOH} \) and \( \mathrm{H}_{3}\mathrm{PO}_{4} \) are reactants that form \( \mathrm{NaH}_{2}\mathrm{PO}_{4} \) and \( \mathrm{H}_{2}\mathrm{O} \) as products.

• Reactions always involve bond breaking and forming.
• Each reaction has specific reactants and products determined by conditions like temperature or concentration.

By understanding chemical reactions, you grasp how substances interact and change, which is crucial for applications ranging from industry to cooking.

Stoichiometry

Stoichiometry is the mathematics behind chemical reactions. It's like a blueprint that tells you exactly how much of each ingredient is needed to craft a new compound. Using stoichiometry, we balance chemical equations and determine amounts of products and reactants.

• It involves using balanced equations to find mole ratios.
• Helps predict how much product forms or how much reactant is needed.

In our reaction, stoichiometry helps calculate the equivalent mass of phosphoric acid by using its proton donation and balancing the number of reactants and products. It’s indispensable for efficiently calculating chemical quantities.