Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

Using Victor Meyer's method, \(0.2 \mathrm{~g}\) of an organic substance displaced \(56 \mathrm{~mL}\) of air at STP. The molecular weight of the compound is (a) 112 (b) 56 (c) 80 (d) 28

Step-by-Step Solution

Answer

The molecular weight of the compound is 80 (c).

Step 1: Understand the Known Quantities

We know that the mass of the organic substance is 0.2 g, and it displaces 56 mL of air at Standard Temperature and Pressure (STP). At STP, 1 mole of gas occupies 22.4 L or 22,400 mL.

Step 2: Convert the Volume to Moles

First, we need to find the number of moles of the displaced gas. The equation is: \[ \text{Number of moles} = \frac{\text{volume in mL}}{22,400} \]Substituting, \[ \text{Number of moles} = \frac{56}{22,400} = 0.0025 \] moles.

Step 3: Calculate Molecular Weight

The molecular weight (M) can be calculated using the formula: \[ M = \frac{\text{mass of substance}}{\text{number of moles}} \]Given the mass is 0.2 g and number of moles is 0.0025, \[ M = \frac{0.2}{0.0025} = 80 \]

Step 4: Conclusion

The molecular weight of the organic compound is found to be 80, which corresponds to option (c).

Key Concepts

Molecular Weight Determination

Molecular weight determination is crucial in chemistry. It allows you to know the amount of substance present. Knowing molecular weights is necessary for chemical reactions and creating compounds. It is expressed in grams per mole (g/mol). You essentially calculate how much one mole of a substance weighs.

The formula generally used is:

• Molecular Weight (M) = \( \frac{\text{mass of the substance}}{\text{number of moles}} \)

Understanding the molecular weight helps chemists in predicting how substances will behave when they react.

Organic Chemistry

Organic chemistry focuses on carbon-containing compounds. These compounds often include hydrogen, oxygen, nitrogen, among other elements. Many everyday substances like plastics, fuels, and medications are studied under organic chemistry.

Walter Meyer contributed to organic chemistry with methods like the Victor Meyer's method, which simplifies determining molecular weight. Knowing molecular weight in organic chemistry can guide you to determine the composition of unknown substances, aiding in breakthroughs in material science and pharmaceuticals.

Gas Displacement Method

The gas displacement method helps measure the volume of gas a sample displaces. This approach is key in Victor Meyer's method for molecular weight determination. It utilizes a substance that vaporizes, pushing gas within a setup like a graduated tube.

This method is useful for volatile liquids where ordinary measurements are challenging. Here's how it generally works:

• A known mass of a substance is heated until it evaporates.
• The vapor displaces air or another gas.
• The displaced gas volume provides data needed for calculating molecular weight.

This method relies on accurate measurement and careful experimental handling.

Standard Temperature and Pressure (STP)

STP is an important concept in physical chemistry. Standard Temperature and Pressure are fixed conditions often used as a reference. **Standard Temperature** is 0°C or 273.15 K. **Standard Pressure** refers to 1 atmosphere (atm) or 101.3 kPa.

At STP, gases have predictable behaviors, making calculations like volume changes much more manageable. For example, one mole of any gas occupies 22.4 liters at STP. This consistency helps in comparing gases and conducting experiments under universal conditions.

Knowing the STP conditions simplifies tasks like the calculation of gas volumes in reaction stoichiometry, often utilized in educational exercises and real-world scenarios.