Question:

At 50 °C, the vapour pressure of pure benzene is 268 torr. The number of moles of non-volatile solute per mole of benzene required to prepare a solution having a vapour pressure of 167 torr at the same temperature is

Updated On: Mar 11, 2025
  • 0.505

  • 0.705

  • 0.605

  • 0.405

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The Correct Option is C

Solution and Explanation

The correct option is: (C): 0.605.

The decrease in vapor pressure when a non-volatile solute is added to a solvent follows Raoult's law. According to Raoult's law, the vapor pressure of a solvent in a solution is proportional to its mole fraction in the solution.

The formula for Raoult's law is:

P_solvent = x_solvent * P°_solvent

Where:

  • P_solvent is the vapor pressure of the solvent in the solution
  • x_solvent is the mole fraction of the solvent in the solution
  • P°_solvent is the vapor pressure of the pure solvent

In this case, you want to find the mole fraction of benzene (the solvent) that results in a vapor pressure of 167 torr at 50 °C. You're given the vapor pressure of pure benzene as 268 torr at the same temperature.

Rearranging the Raoult's law equation to solve for x_solvent:

x_solvent = P_solvent / P°_solvent

Plug in the values:

x_solvent = 167 torr / 268 torr ≈ 0.6231

Now, the mole fraction of the solute (non-volatile solute) can be calculated as:

x_solute = 1 - x_solvent ≈ 1 - 0.6231 ≈ 0.3769

The number of moles of non-volatile solute per mole of benzene can be calculated using the mole fractions:

Moles of solute / Moles of benzene = x_solute / x_solvent

Moles of solute / 1 = 0.3769 / 0.6231

Moles of solute ≈ 0.605

So, the answer is approximately 0.605

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Concepts Used:

Solutions

A solution is a homogeneous mixture of two or more components in which the particle size is smaller than 1 nm.

For example, salt and sugar is a good illustration of a solution. A solution can be categorized into several components.

Types of Solutions:

The solutions can be classified into three types:

  • Solid Solutions - In these solutions, the solvent is in a Solid-state.
  • Liquid Solutions- In these solutions, the solvent is in a Liquid state.
  • Gaseous Solutions - In these solutions, the solvent is in a Gaseous state.

On the basis of the amount of solute dissolved in a solvent, solutions are divided into the following types:

  1. Unsaturated Solution- A solution in which more solute can be dissolved without raising the temperature of the solution is known as an unsaturated solution.
  2. Saturated Solution- A solution in which no solute can be dissolved after reaching a certain amount of temperature is known as an unsaturated saturated solution.
  3. Supersaturated Solution- A solution that contains more solute than the maximum amount at a certain temperature is known as a supersaturated solution.