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

(c) molecular mass/acidity

Step 1: Understanding the Concept of Equivalent Mass

The equivalent mass of a substance is the mass that combines with or displaces 1 mole of hydrogen atoms. For acids and bases, it is related to the amount of replaceable hydrogen ions (acidity) or hydroxide ions (basicity), respectively.

Step 2: Recognizing the Definition of Acidity and Basicity

Acidity refers to the number of hydrogen ions an acid can donate, while basicity refers to the number of hydroxide ions a base can donate.

Step 3: Calculating Equivalent Mass for Acids

For an acid, the equivalent mass is calculated by dividing its molecular mass by its acidity. This is because the equivalent mass is the mass required to provide one mole of replaceable hydrogen ions: equivalent mass = \( \frac{\text{molecular mass}}{\text{acidity}} \).

Step 4: Identifying the Correct Option for Acids

Given the choices, the correct option for calculating the equivalent mass of an acid based on its molecular mass and acidity is (c) molecular mass/acidity.

Key Concepts

Molecular Mass

Molecular mass refers to the mass of a given molecule, calculated as the sum of the atomic masses of all the atoms present in the molecule. To find the molecular mass, you would add up the atomic masses of each constituent atom as found on the periodic table. The atomic mass of an element is typically measured in atomic mass units (amu).

Here's a simplified process of determining the molecular mass:

• Identify each type of atom in the molecule.
• Refer to the periodic table to find the atomic mass of each atom.
• Multiply the atomic mass of each type of atom by the number of atoms of that type in the molecule.
• Add all these values together to get the molecular mass.

Molecular mass is a crucial concept because it allows chemists to predict how compounds interact in chemical reactions, determine stoichiometric relationships, and calculate equivalent masses.

Acidity

Acidity is a measure of how many hydrogen ions (H⁺) an acid can donate when it dissolves in water. In chemistry, an acid is a substance that can donate protons (H⁺ ions) or accept an electron pair in reactions. Acidity involves several key points:

• The more H⁺ ions an acid can donate, the stronger the acid is considered to be.
• Acidity is often expressed in terms of pH, a logarithmic scale where lower values correspond to higher acidity.
• In acid-base reactions, the acidity influences how aggressively the acid will react with bases.

When determining equivalent mass, the molecule's acidity is important. To calculate an acid's equivalent mass, the molecular mass is divided by its acidity. This calculation gives the amount of the acid needed to donate one mole of hydrogen ions.

Basicity

Basicity, on the other hand, indicates the capacity of a base to accept hydrogen ions or donate hydroxide ions (OH⁻). Strong bases can readily accept hydrogen ions or provide hydroxide ions in aqueous solutions. Important aspects of basicity include:

• The basicity of a compound tells us how many hydroxide ions it can produce in a solution.
• Just like acidity, basicity can be measured with a pH scale, but higher values indicate greater basicity.
• The strength of a base is determined by its ability to dissociate in water and form hydroxide ions.

When calculating the equivalent mass of a base, the molecular mass is divided by its basicity. This process assumes that the basicity represents the number of moles of replaceable hydroxide ions per mole of base.