Problem 52 - A Complete Resource Book in Chemistry for JEE Main

Question

\(0.1914 \mathrm{~g}\) of an organic acid is dissolved in about 20 \(\mathrm{mL}\) of water \(25 \mathrm{~mL}\) of \(0.12 \mathrm{~N} \mathrm{NaOH}\) is required for the complete neutralization of the acid solution. The equivalent weight of the acid is (a) \(63.8\) (b) \(64.0\) (c) \(65.8\) (d) \(66.2\)

Step-by-Step Solution

Answer

The equivalent weight of the acid is 63.8, option (a).

Step 1: Understand the Given Problem

This problem involves determining the equivalent weight of an unknown organic acid using a titration method where a known volume and normality of sodium hydroxide (NaOH) is used to neutralize a given mass of the acid.

Step 2: Identify the Relevant Formula

To find the equivalent weight of the acid, we use the formula: Equivalent weight of acid = (Mass of acid) / (Equivalent of NaOH used).

Step 3: Calculate Equivalents of NaOH

We calculate the equivalents of NaOH used during the titration: Equivalents = Normality \( \times \) Volume in liters, hence \(0.12\, \text{N} \times 0.025\, \text{L} = 0.003\, \text{equivalents}\).

Step 4: Solve for Equivalent Weight of the Acid

Using the formula from Step 2, substitute the values: Equivalent weight of acid = \( \frac{0.1914\, \text{g}}{0.003} = 63.8 \, \text{g/equiv}\).

Step 5: Determine the Correct Option

Compare the calculated equivalent weight of \(63.8\, \text{g/equiv}\) to the options provided: (a) \(63.8\), (b) \(64.0\), (c) \(65.8\), (d) \(66.2\). The correct choice for the equivalent weight is option (a) \(63.8\).

Key Concepts

Understanding Organic Acid Titration

Titration is a method used to determine the concentration of a substance in a solution by reacting it with a solution of known concentration, called a titrant. In organic acid titration, an acid with an unknown concentration is reacted with a base of known concentration. This titration process helps in determining the equivalent weight of the acid.

During titration:

• A known volume of the base (like NaOH) is added gradually to the acid solution.
• The addition continues until the reaction reaches its endpoint, signifying complete neutralization.

Complete neutralization is when the acid and the base have reacted fully according to their stoichiometric ratios. Several indicators like phenolphthalein or methyl orange are used to signal this endpoint, often by a visible color change.
This process helps in accurately calculating the equivalent weight, especially essential when dealing with organic acids whose chemical structure might be unknown or complex.

The Role of Normality in Chemistry

Normality (N) is a measure of concentration equivalent to molarity but adjusted for the reactive capacity of the solute. Unlike molarity, which focuses solely on the number of moles of solute per liter of solution, normality considers the number of equivalents per liter.
In the context of acid-base reactions, like the one seen in titration, normality helps account for the number of hydrogen ions (protons) that each molecule of an acid can donate. Similarly, it reflects the capacity of a base to neutralize hydrogen ions.

For example:

• A 0.12 N NaOH solution means there are 0.12 equivalents of base per liter, illustrating how many protons it can neutralize.
• This comes into play in calculating the equivalents used during titration, which is crucial for determining the equivalent weight of an acid.

Using normality allows us to directly use the formula for equivalents, making calculations straightforward when dealing with acids and bases.

Understanding Neutralization Reactions

Neutralization is a chemical reaction between an acid and a base that results in the formation of water and a salt. It occurs when the number of moles of H\(^+\) ions from the acid equals the number of moles of OH\(^-\) ions provided by the base.
This reaction type is crucial in titrations, forming the basis for neutralizing acids with bases like sodium hydroxide. For instance, when NaOH (a strong base) neutralizes an acid, the reaction can be illustrated as:

• Acid + Base → Salt + Water
• Example: HCl + NaOH → NaCl + H\(_2\)O

The reaction shows how the acid donates protons that the base accepts, resulting in a neutral solution.

Neutralization helps us find the equivalent points where the titrant neutralizes the analyte. This endpoint's measurement allows the calculation of equivalent weights, offering insights into the amount of substance involved in the reaction.
Thus, understanding this principle is crucial for determining specific properties like equivalent weight in acid-base titrations.