Which of the following units is useful in relating concentration of solution with its vapour pressure?

Options

A. Mole fraction
B. Parts per million
C. Mass percentage
D. Molality

Answer

To solve the question of which unit is useful in relating the concentration of a solution with its vapor pressure, we can follow these steps:

### Step 1: Understand the Relationship
We need to understand how concentration and vapor pressure are related. According to Henry's Law, the partial pressure of a gas above a solution is directly proportional to the mole fraction of that gas in the solution.

### Step 2: Write Down Henry's Law
Henry's Law can be expressed mathematically as:
\[ P = k_H \cdot x \]
Where:
- \( P \) is the partial pressure of the gas,
- \( k_H \) is Henry's constant (specific to the gas and solvent),
- \( x \) is the mole fraction of the gas in the solution.

### Step 3: Identify the Relevant Unit
From the equation, we see that the mole fraction \( x \) is the unit that relates the concentration of the solution with its vapor pressure. The mole fraction is a dimensionless quantity that represents the ratio of the number of moles of a component to the total number of moles in the solution.

### Step 4: Conclusion
Thus, the unit that is useful in relating the concentration of a solution with its vapor pressure is the **mole fraction**.

### Final Answer
The correct answer is **mole fraction**.

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