Unit of lowering of vapour pressure is

Options

A. atm
B. `Nm^(-1)`
C. Unitless
D. `Nm^(2)`

Answer

To determine the unit of lowering of vapor pressure, we can follow these steps:

### Step 1: Understand the Concept of Vapor Pressure
- Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid or solid phase at a given temperature. When a solute is added to a solvent, the vapor pressure of the solvent decreases.

### Step 2: Define Lowering of Vapor Pressure
- The lowering of vapor pressure (ΔP) can be defined as the difference between the vapor pressure of the pure solvent (P₀) and the vapor pressure of the solvent with solute (P):
\[ \Delta P = P₀ - P \]

### Step 3: Identify the Units of Vapor Pressure
- The units of vapor pressure are the same as those of pressure. Common units of pressure include:
- Atmospheres (atm)
- Pascals (Pa)
- Newtons per square meter (N/m²)

### Step 4: Analyze the Options Given
- The options provided are:
1. atm
2. N/m²
3. Unitless
4. N·m²

### Step 5: Determine the Correct Unit
- Since ΔP is a difference in pressure, it will have the same units as pressure itself. Therefore, the unit of lowering of vapor pressure is:
- atm (option 1)

### Step 6: Eliminate Incorrect Options
- Option 2 (N/m²) is a valid unit of pressure, but since we are looking for the most common unit, atm is preferred.
- Option 3 (unitless) is incorrect because ΔP has dimensions.
- Option 4 (N·m²) is also incorrect because it does not represent a unit of pressure.

### Conclusion
- The correct answer is **atm**.

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