📘 Understanding Vapour Pressure in a Binary Liquid–Liquid Solution

 Here is a student-friendly and exam-oriented explanation of the paragraph you provided, suitable for Class 12 Chemistry (CBSE/RBSE):

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📘 Understanding Vapour Pressure in a Binary Liquid–Liquid Solution

Let’s consider a binary solution (a solution with two components), where both components are volatile liquids. We'll refer to them as:

• Component 1

• Component 2

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🧪 What Happens in a Closed Container?

• When this solution is placed in a closed vessel, both components will begin to evaporate.

• After some time, a state of dynamic equilibrium is reached between the vapour phase and the liquid phase.

This means:

• Molecules of both components evaporate and condense at the same rate.

• The system becomes stable, and the vapour pressure remains constant.

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💡 Vapour Pressure at Equilibrium:

Let’s define:

• $p_1$: Partial vapour pressure of component 1

• $p_2$: Partial vapour pressure of component 2

• $p_{\text{total}}$: Total vapour pressure of the solution

• $x_1$: Mole fraction of component 1 in the liquid phase

• $x_2$: Mole fraction of component 2 in the liquid phase

Then:

$$p_{\text{total}} = p_1 + p_2$$

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🔗 Mole Fraction Relation (Raoult’s Law - to be introduced next):

These partial pressures are directly related to the mole fractions of the two components in the liquid phase.

This sets the stage for applying Raoult’s Law, which gives a quantitative relationship between mole fraction and vapour pressure in ideal solutions.

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📝 Summary:

• In a binary solution of two volatile liquids, both components evaporate.

• In a closed vessel, equilibrium is established.

• Each component contributes to the total vapour pressure according to its mole fraction.

• This concept leads us to Raoult’s Law, which we'll use to mathematically express these relationships.

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