Question

Select correct statements/s from the following:

$\boxed{\text{ }}$ A solution of HCI in water follows Henry's Law

$\boxed{\text{ }}$ A mixture of ethanol and acetone behave as a non ideal solution with positive deviation from the Raoult's law

$\boxed{\text{ }}$ A mixture of $CH_2Cl_2$ and $CHCl_3$ behave ideally and obey Raoult's law

$\boxed{\text{ }}$ Vapour pressure of a solvent always decreases on intermixing irrespective of nature of the solute dissolved

• A. A solution of HCI in water follows Henry's Law
• B. A mixture of ethanol and acetone behave as a non ideal solution with positive deviation from the Raoult's law
• C. A mixture of $CH_2Cl_2$ and $CHCl_3$ behave ideally and obey Raoult's law
• D. Vapour pressure of a solvent always decreases on intermixing irrespective of nature of the solute dissolved

Answer 1

Answer: (B), (D)

A mixture of ethanol and acetone behave as a non ideal solution with positive deviation from the Raoult's law, Vapour pressure of a solvent always decreases on intermixing irrespective of nature of the solute dissolved

Answer 2

Let's analyze each statement:

1. A solution of HCl in water follows Henry's Law: Henry's law states that the partial pressure of a gas above a solution is proportional to its mole fraction in the solution. It is applicable to dilute solutions of gases that do not undergo significant chemical reactions or dissociation/association in the solvent. HCl is a gas that dissolves in water and undergoes nearly complete dissociation into H⁺ and Cl⁻ ions. This strong chemical interaction and dissociation mean that the solubility of HCl in water does not follow Henry's law. Therefore, this statement is incorrect.

2. A mixture of ethanol and acetone behave as a non ideal solution with positive deviation from the Raoult's law: Ethanol molecules are associated through hydrogen bonding. Acetone molecules have dipole-dipole interactions. When ethanol and acetone are mixed, some of the hydrogen bonds in ethanol are broken, and the interactions between ethanol and acetone molecules are weaker than the hydrogen bonding in pure ethanol. Weaker intermolecular forces in the solution compared to the pure components lead to higher vapor pressure than predicted by Raoult's law. This is characteristic of a positive deviation from Raoult's law. Therefore, this statement is correct.

3. A mixture of $CH_2Cl_2$ and $CHCl_3$ behave ideally and obey Raoult's law: For a solution to be ideal, the intermolecular interactions between solute-solvent (A-B) should be similar to the interactions between solute-solute (A-A) and solvent-solvent (B-B). Dichloromethane ($CH_2Cl_2$) and chloroform ($CHCl_3$) are both polar molecules. Chloroform has a slightly acidic hydrogen atom, and dichloromethane has chlorine atoms with lone pairs of electrons. A weak hydrogen bond can form between the hydrogen of chloroform and a chlorine of dichloromethane (or chloroform itself). This interaction between $CH_2Cl_2$ and $CHCl_3$ is stronger than the average of the interactions in pure $CH_2Cl_2$ and pure $CHCl_3$. Stronger intermolecular forces in the solution lead to lower vapor pressure than predicted by Raoult's law, resulting in a negative deviation. Therefore, this statement is incorrect; the mixture behaves as a non-ideal solution with negative deviation.

4. Vapour pressure of a solvent always decreases on intermixing irrespective of nature of the solute dissolved: This statement refers to the partial vapor pressure of the solvent. When a solute is dissolved in a solvent, the mole fraction of the solvent ($X_{solvent}$) in the solution is less than 1. According to Raoult's law, the partial vapor pressure of the solvent in an ideal solution is $P_{solvent} = X_{solvent} P^0_{solvent}$, where $P^0_{solvent}$ is the vapor pressure of the pure solvent. Since $X_{solvent} < 1$, $P_{solvent} < P^0_{solvent}$. This represents a decrease in the partial vapor pressure of the solvent. Even in non-ideal solutions, the chemical potential of the solvent in the solution is lower than that of the pure solvent at the same temperature (due to the entropy of mixing), which implies that its partial vapor pressure is lower than the vapor pressure of the pure solvent. This holds true for any solute dissolved in a solvent. Therefore, this statement is correct.

Based on the analysis, statements 2 and 4 are correct.

Explanation of the solution: Statement 1 is incorrect because HCl dissociates in water, violating the assumptions of Henry's law. Statement 2 is correct because the weaker interactions between ethanol and acetone compared to hydrogen bonding in pure ethanol cause positive deviation from Raoult's law. Statement 3 is incorrect because a weak hydrogen bond between $CH_2Cl_2$ and $CHCl_3$ leads to stronger interactions and negative deviation from Raoult's law. Statement 4 is correct because adding a solute reduces the mole fraction of the solvent, leading to a decrease in the partial vapor pressure of the solvent compared to its pure state, as described by Raoult's law and the concept of chemical potential lowering upon mixing.

Answer: Statements 2 and 4 are correct.