Ideal and Non-Ideal Solutions (NCERT) - NEET Chemistry Class 12 Free MCQ

Concepts to Use:

We will apply Raoult’s Law and Dalton’s Law.

Step 1: Raoult’s Law

Raoult’s Law gives the partial pressure of each component in the liquid mixture as:

• pA = xA_liquid × pA⁰

• pB = xB_liquid × pB⁰

Here:

• pA⁰ > pB⁰ because component A is more volatile.

Step 2: Dalton’s Law of Partial Pressures

Dalton’s Law tells us how to find the mole fraction of A in the vapour phase:

yA = pA / (pA + pB)
Substitute pA and pB from Raoult’s Law:

yA = (xA_liquid × pA⁰) / (xA_liquid × pA⁰ + xB_liquid × pB⁰)

Step 3: Compare yA and xA_liquid

Since pA⁰ > pB⁰, the numerator in the above expression is weighted more heavily in favor of A. That means:

yA > xA_liquid

So, the vapour phase is richer in the more volatile component A than the liquid phase.

Step 4: What About Component B?

If yA > xA_liquid, then it must be true that:

yB < xB_liquid
(since yA + yB = 1 and xA + xB = 1)

Therefore, the vapour phase is poorer in B, and the liquid phase is richer in B.

Conclusion:

This implies that:

(xA)_liquid < (xB)_vapour

Correct Option: c)

Why Not the Other Options?

• Option a: Incorrect. Compositions in liquid and vapour are not the same for an ideal solution.

• Option b: Incorrect. Liquid phase is actually poorer in the more volatile component A.

• Option d: Incorrect. The relationship is clearly defined by Raoult’s and Dalton’s laws.

Final Answer:

c) (xA)_liquid < (xB)_vapour

For an ideal solution, ΔH and Delta ΔV for mixing should be zero. P_Total = p_A + p_B and A − A, B − B and A − B interactions are nearly same.

Azeotropes are formed by the solutions which show deviations from ideal behaviour.

Acetone + ethanol is an example of solutions showing positive deviation from Raoult's law. Since acetone-ethanol attractions are weaker than acetone - acetone and ethanol-ethanol attractions.

CH₃COCH₃ + CHCl₃ is an example of solution showing negative deviation from Raoult's law since A-B attractions (acetone + chloroform) are more than A - A (acetone -acetone) and B - B( chloroform-chloroform) attractions.

Acetone + ethyl alcohol solution shows positive deviation while acetone + chloroform shows negative deviation.
Other examples:
Positive deviations - Acetone + ethyl alcohol,
acetone + benzene, water + ethyl alcohol
Negative deviations - Nitric acid + water,
benzene + chloroform

When acetone and chloroform are mixed together, hydrogen bonds are formed between them which increases intermolecular interactions hence decreases the vapour pressure showing negative deviation.

(iii) and (iv) will form ideal solutions hence do not form azeotropes. Azeotropes have same composition in liquid and vapour form when distilled.

To determine which binary mixtures will have the same composition in the liquid and vapor phases, we need to identify mixtures that form ideal solutions. In an ideal solution, the composition of the liquid phase and the vapor phase is the same at equilibrium.

Here’s a brief analysis of each mixture:

1. Ethanol + Chloroform:

   • This mixture does not behave ideally due to strong hydrogen bonding interactions between ethanol and chloroform, which can cause deviations from Raoult's Law.

2. Nitric Acid + Water:

   • Nitric acid and water form a non-ideal solution. Nitric acid forms strong hydrogen bonds with water, resulting in significant deviations from ideal behavior. Thus, the composition in the liquid and vapor phases will not be the same.

3. Benzene + Toluene:

   • Benzene and toluene form an ideal solution. The interactions between benzene and toluene are similar, and thus the composition of the liquid and vapor phases will be the same.

4. Ethyl Chloride + Ethyl Bromide:

   • Ethyl chloride and ethyl bromide also form an ideal solution. The interactions between these two similar substances lead to minimal deviations from ideal behavior.

Based on the above analyses, the mixtures that will have the same composition in the liquid and vapor phases are:

2. (iii) and (iv)

So the correct answer is:

2. (iii) and (iv)

When acetone and chloroform are mixed together, a hydrogen bond is formed between them which increases intermolecular interactions. Hence, A − B interactions are stronger than A − A and A − B interactions.

Due to stronger intermolecular interactions in acetone and chloroform lesser number of molecules vaporise resulting in low vapour pressure and high boiling point.