Problem 2 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Molecular weight of acids can be detemined by (a) Silver salt method (b) Volumetric method (c) Plants chloride method (d) Victor Meyer's method

Step-by-Step Solution

Answer

The molecular weight of acids can be determined by the silver salt method.

Step 1: Understanding the Question

The question is asking which methods can be used to determine the molecular weight of acids. Several methods in chemistry are used specifically for determining molecular weights of certain types of compounds.

Step 2: Identifying Each Method

Let's identify what each method is used for: 1. **Silver salt method** is used to determine the molecular weight of carboxylic acids by converting them into their silver salts. 2. **Volumetric method** involves titration which can be used to find the equivalent weight but not directly for molecular weight. 3. **Plants chloride method** doesn't directly relate to determining molecular weights for acids. 4. **Victor Meyer's method** is traditionally used for determining the molecular weight of volatile substances.

Step 3: Matching Methods with Acids

For determining the molecular weight of acids, typically the silver salt method is most directly used among the given options, as it specifically involves converting carboxylic acids into salts to facilitate this determination.

Key Concepts

Silver Salt Method

The Silver Salt Method is a classical technique for determining the molecular weight of carboxylic acids. This method involves converting the carboxylic acid into its corresponding silver salt. Silver salts are typically formed due to the reaction between carboxylic acids and silver nitrate. The reason this method is effective lies in the predictable nature of this chemical conversion.

Here's how it works:

• First, a sample of the carboxylic acid is dissolved in water or an appropriate solvent.
• Silver nitrate is then added, causing the formation of a precipitate.
• This precipitate is then collected, dried, and weighed.

The mass of the silver salt generated can be used to calculate the molecular weight of the original acid. By knowing the stoichiometry of the acid and silver salt, chemists can derive the precise molecular weight of the acid sample. This method is particularly helpful for non-volatile acids that are hard to measure through other techniques.

Victor Meyer's Method

Victor Meyer's Method is an established approach for determining the molecular weight of volatile substances, not exclusively for acids. This method relies on gauging the change in volume before and after vaporization of a substance within a confined space, using gases laws as a foundation.

In a typical procedure:

• The substance whose molecular weight is to be determined is first evaporated.
• It is heated in a device known as a Victor Meyer apparatus. This apparatus allows the observation of the volume of vapor generated.
• The expansion and displacement of a non-reactive gas, such as air or nitrogen, are then measured.

The volume of the displaced gas directly correlates to the volume of the vapor produced under set temperature and pressure conditions. By employing the Ideal Gas Law formula, chemists can calculate the molecular weight of the evaporated compound. While not chiefly used for carboxylic acids, Victor Meyer's method remains a crucial tool for substances that can be vaporized.

Carboxylic Acids

Carboxylic acids are organic compounds characterized by the presence of the carboxyl group \(-COOH\). They are prevalent in nature and found in various consumer products. Carboxylic acids exhibit several recognizable properties:

• They are generally polar, making them soluble in water to some extent.
• These compounds can form hydrogen bonds due to the polar -OH group.
• They usually have higher boiling points than alcohols, aldehydes, and ketones of similar molecular weights.

In the context of molecular weight determination, carboxylic acids can be studied using the Silver Salt Method, among others.

Many carboxylic acids exist as key components in the biochemical world, such as acetic acid, the active ingredient in vinegar, and citric acid, found within citrus fruits. Understanding their molecular weights is vital as it informs chemists about their potential reactivity and applications.