Introduction

The vapour pressure of a liquid is the pressure exerted by its vapour when the liquid and its vapour are in dynamic equilibrium. In a solution, the vapour pressure depends on the mole fraction of each component present. The mole fraction is the ratio of the number of moles of a component to the total number of moles in the solution. When the mole fraction of component 1 increases, the total vapour pressure over the solution can either increase or decrease, depending on the vapour pressures of the pure components.

Explanation

1. If the vapour pressure of component 1 is lower than component 2:
- The vapour pressure of a component is a measure of its tendency to escape from the liquid phase and enter the vapour phase. If component 1 has a lower vapour pressure than component 2, it means that component 1 has a stronger intermolecular forces or is heavier than component 2.
- When the mole fraction of component 1 increases, the concentration of component 2 decreases. As a result, the overall intermolecular forces in the solution decrease, and the average kinetic energy of the molecules increases.
- The increased kinetic energy leads to an increased rate of evaporation of component 1 molecules from the liquid phase into the vapour phase. This results in an increase in the total vapour pressure over the solution.

2. If the vapour pressure of component 1 is higher than component 2:
- If component 1 has a higher vapour pressure than component 2, it means that component 1 has weaker intermolecular forces or is lighter than component 2.
- When the mole fraction of component 1 increases, the concentration of component 2 decreases. This leads to a decrease in the overall intermolecular forces in the solution, and an increase in the average kinetic energy of the molecules.
- The increased kinetic energy leads to an increased rate of evaporation of component 1 molecules from the liquid phase into the vapour phase. However, at the same time, the higher vapour pressure of component 1 means that there is a greater tendency for component 1 molecules to condense back into the liquid phase.
- As a result, the net effect of increasing the mole fraction of component 1 is a decrease in the total vapour pressure over the solution.

Conclusion

The change in the total vapour pressure over a solution when the mole fraction of component 1 increases depends on the relative vapour pressures of the pure components. If the vapour pressure of component 1 is lower than component 2, the total vapour pressure increases with increasing mole fraction of component 1. On the other hand, if the vapour pressure of component 1 is higher than component 2, the total vapour pressure decreases with increasing mole fraction of component 1.