Problem 19 - A Complete Resource Book in Chemistry for JEE Main

Question

The equivalent mass of an acid is equal to (a) molecular mass \(\times\) acidity (b) molecular mass \(\times\) basicity (c) molecular mass/acidity (d) molecular mass/basicity

Step-by-Step Solution

Answer

The equivalent mass of an acid is (d) molecular mass/basicity.

Step 1: Understanding the Problem

To find the equivalent mass of an acid, we need to know the concept of equivalent mass. The equivalent mass of a substance is the mass that combines with or displaces 1 mole of hydrogen atoms (1.008 grams). For acids, equivalent mass is determined using the molecular mass and basicity.

Step 2: Defining Basicity

Basicity refers to the number of moles of replaceable hydrogens in one mole of acid. It is used in calculating the equivalent mass of an acid.

Step 3: Formula for Equivalent Mass of an Acid

The equivalent mass of an acid is given by the formula: \[ \text{Equivalent Mass} = \frac{\text{Molecular Mass}}{\text{Basicity}} \]

Step 4: Identifying the Correct Option

Based on the formula, the equivalent mass of an acid is the molecular mass divided by its basicity. Thus, the correct option is (d) molecular mass/basicity.

Key Concepts

Acidity

Acidity is a fundamental concept in chemistry, especially when understanding acids and their behaviors. Acidity refers to the strength or concentration of an acid in a solution. It is often measured by the ability of an acid to donate protons (\(\text{H}^+\) ions) to a base in a reaction. Some key points to consider about acidity are:

• Acids have a sour taste, which is one of the most common properties associated with acidity.
• Acidity levels can be measured using the pH scale, where lower pH values indicate higher acidity.
• Strong acids like hydrochloric acid (\(\text{HCl}\)) completely dissociate in water, whereas weak acids do not.

To understand how acidity plays a role in determining the equivalent mass of an acid, it's essential to recognize how acidity affects reaction outcomes, especially in chemical titrations where acid-base neutralization occurs. Although not directly used to calculate the equivalent mass of an acid, acidity provides context for the acid's behavior in various reactions.

Basicity

Basicity is another essential concept when analyzing acids, as it plays a crucial role in calculating the equivalent mass. Basicity refers to the number of hydrogen ions (\(\text{H}^+\)) an acid can donate or the number of replaceable hydrogen atoms in a molecule. Important aspects of basicity include:

• In terms of acids, basicity is the number of moles of \(\text{H}^+\) ions supplied by one mole of acid.
• Monoprotic acids, like \(\text{HCl}\), have a basicity of 1 because they release one \(\text{H}^+\) per molecule.
• Polyprotic acids, such as sulfuric acid (\(\text{H}_2\text{SO}_4\)), have higher basicities. \(\text{H}_2\text{SO}_4\) has a basicity of 2, as it can donate two \(\text{H}^+\) ions.

The calculation of equivalent mass for an acid depends on its basicity. The formula, \( \text{Equivalent Mass} = \frac{\text{Molecular Mass}}{\text{Basicity}} \), relies on knowing this number since it describes how many \(\text{H}^+\) ions are involved in a reaction, affecting the proportionate mass used in equivalencies.

Molecular Mass

Molecular mass, also referred to as molecular weight, is the sum of the atomic masses of all atoms in a molecule. Knowing the molecular mass is vital when calculating the equivalent mass of substances as it interrelates with other properties like basicity and acidity. Crucial points about molecular mass include:

• It is usually expressed in atomic mass units (amu), where one amu is \(1/12\) the mass of a carbon-12 atom.
• Molecular mass provides a way to quantify the amount of a substance in moles.
• Accurate determination of molecular mass allows for precise stoichiometric calculations in chemical reactions.

In the context of calculating equivalent mass of an acid, the molecular mass is divided by the acid's basicity to find the mass that participates equivalently in reactions with hydrogen ions. This highlights how the understanding of molecular mass plays an integral role in determining chemical proportions and reaction predictions.