Q.Which of the following statements is false?

The key idea is Henry’s law (for gas solubility in a liquid), but the question tests general colligative properties and osmosis. Let’s check each statement:

• (i) Both atmospheric pressure and osmotic pressure are measured in atm, Pa, or bar — same units. True.
• (ii) Reverse osmosis applies external pressure to force solvent from a higher-solute (higher osmotic pressure) region to a lower-solute region. The given direction is opposite. False. …

Let’s understand why. Colligative properties — like osmotic pressure, vapour pressure lowering, boiling point elevation, and freezing point depression — depend only on the number of solute particles, not their identity. Each option tests a specific concept. We’ll go through them one by one.

1. Option (i): Units of atmospheric pressure and osmotic pressure are the same.

   Osmotic pressure ($\pi$) is defined by the van’t Hoff equation: $\pi = iCRT$, where $C$ is molar concentration, $R$ is the gas constant, and $T$ is temperature. Since $R$ has units of $\text{L·atm·mol}^{-1}\text{K}^{-1}$ (or $\text{J·mol}^{-1}\text{K}^{-1}$), the product $CRT$ gives pressure units — typically atm or Pa. Atmospheric pressure is also measured in atm or Pa. So yes, the units are identical. This statement is true.

2. Option (ii): In reverse osmosis, solvent molecules move through a semipermeable membrane from a region of lower concentration of solute to a region of higher concentration.

   Osmosis naturally moves solvent from a region of lower solute concentration (higher solvent concentration) to higher solute concentration (lower solvent concentration). Reverse osmosis is the opposite: we apply an external pressure greater than the osmotic pressure to force solvent from the higher solute concentration side to the lower solute concentration side. So the direction described in (ii) — from lower to higher concentration — is actually the direction of normal osmosis, not reverse osmosis. Therefore, this statement is false.

   Watch out

   A common mistake is to confuse the direction of normal osmosis with reverse osmosis. In reverse osmosis, the solvent moves against its natural gradient — from higher solute concentration to lower solute concentration — only because an external pressure is applied.

3. Option (iii): The value of molal depression constant depends on nature of solvent. …

Method: Conceptual Elimination with Henry's Law Context

Method name: Conceptual elimination using definitions and dimensional analysis

Step 1 — Recall Henry's Law and its context

Henry's Law states:

At constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.

Mathematically:

where $P$ is partial pressure, $K_H$ is Henry's constant, and $x$ is mole fraction of the gas in solution.

This law is related to gas solubility, not directly to the four statements given — but it helps you recall the nature of pressure, concentration, and dimensionless quantities.

Step 2 — Analyse each statement one by one

(i) Units of atmospheric pressure and osmotic pressure are the same.

• Atmospheric pressure: $\text{Pa}$ or $\text{atm}$ or $\text{bar}$
• Osmotic pressure ($\pi = iCRT$): also has units of pressure ($\text{Pa}$, $\text{atm}$) ✓ True

(ii) In reverse osmosis, solvent molecules move through a semipermeable membrane from a region of lower concentration of solute to a region of higher concentration.

• Reverse osmosis: opposite of natural osmosis.
• In osmosis: solvent moves from low solute concentration to high solute concentration. …

Here are the common mistakes students make on this Henry’s Law / colligative properties question, and how to avoid each.

Mistake 1: Confusing Henry’s Law with Osmotic Pressure / Reverse Osmosis

The error: Students see “Henry’s Law” in the topic heading and assume all options are about gas solubility. They then misread option (ii) about reverse osmosis, thinking it describes Henry’s Law (gas dissolving in liquid). This leads to guessing the wrong false statement.

How to avoid:

• Read each option independently. This question tests multiple concepts from the “Solutions” chapter, not just Henry’s Law.
• For reverse osmosis: remember the solvent moves from low solute concentration (high solvent concentration) to high solute concentration (low solvent concentration). That’s exactly what option (ii) says — so it is true.
• Key fact: Reverse osmosis is the opposite of osmosis; it requires external pressure, but the direction of solvent flow is still from dilute to concentrated side.

Mistake 2: Thinking Osmotic Pressure and Atmospheric Pressure Have Different Units

The error: Students think osmotic pressure is a “special” pressure with different units (like $\pi = iCRT$ in atm or bar), while atmospheric pressure is measured in mm Hg or Pa. They conclude units differ.

How to avoid:

• Both are pressures. Osmotic pressure ($\pi$) is measured in atm, bar, Pa, or mm Hg — exactly the same units as atmospheric pressure.
• Memorise: All forms of pressure (osmotic, atmospheric, vapour pressure) share the same units. Option (i) is true.

Mistake 3: Forgetting That Molal Depression Constant ($K_f$) Depends Only on the Solvent

The error: Students think $K_f$ depends on the solute because the freezing point depression $\Delta T_f = K_f \cdot m$ involves molality of solute. They mark option (iii) as false.

How to avoid:

• $K_f$ is a solvent property. It depends on the solvent’s molar mass, freezing point, and enthalpy of fusion — not on the solute.
• Example: $K_f$ for water is $1.86 \, \text{K kg mol}^{-1}$, for benzene it’s $5.12 \, \text{K kg mol}^{-1}$. The solute changes $\Delta T_f$, but $K_f$ stays fixed for a given solvent.
• Option (iii) is true.

Mistake 4: Thinking Relative Lowering of Vapour Pressure Has Units

The error: Students see “vapour pressure” and assume the quantity must have pressure units (atm, mm Hg). They mark option (iv) as false. …