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

Explanation

To solve the question regarding 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 between vapor pressure and concentration
Vapor pressure of a solution is influenced by the concentration of the solute in the solution. According to Raoult's Law, the vapor pressure of a solvent in a solution is directly related to the mole fraction of the solvent.

### Step 2: Review the options given
The options provided are:
a. mole fraction
b. parts per million
c. percentage
d. mass percentage
e. molality

### Step 3: Analyze each option
- **Mole fraction**: This is the ratio of the number of moles of a component to the total number of moles of all components in the solution. It is directly related to vapor pressure according to Raoult's Law.

- **Parts per million (ppm)**: This is generally used for very dilute solutions and is not directly related to vapor pressure in a straightforward manner.

- **Percentage**: This can refer to weight percentage or volume percentage, but it does not provide a direct relationship with vapor pressure.

- **Mass percentage**: Similar to percentage, this does not directly relate to vapor pressure.

- **Molality**: This is the number of moles of solute per kilogram of solvent and does not directly relate to vapor pressure.

### Step 4: Conclusion
Since the vapor pressure of a solution is directly proportional to the mole fraction of the solvent, the correct answer is **a. mole fraction**.

### Final Answer
**a. mole fraction**