Question

What is the temperature of the water when the vapour pressure is $30$ kPa?
(A) $20{}^\circ C.$
(B) $100{}^\circ C.$
(C) $70{}^\circ C.$
(D) $40{}^\circ C.$

Answer 1

We know that the vapour pressure is a measure of the tendency of a material to change from liquid state to gaseous or vapour state and the vapour pressure increases with increase in the temperature. If a substance has high vapour pressure at normal temperature, then the substance is volatile in nature.

Complete answer:
In order to answer this question, you must enlist all the concepts of Solutions and go through all of Raoult’s laws and must use the concept of vapour pressure. For solution, firstly evaluate the mole fraction of the component from the given data and then calculate the vapour pressure of the solution, and then the vapour pressure of the solution is the required quantity
Vapour pressure is a measure of the tendency of a material to change into the gaseous or vapour state, and it increases with temperature. Raoul’s law states that the vapor pressure of a solvent above a solution is equal to the vapor pressure of the pure solvent at the same temperature scaled by the mole fraction of the solvent present. For a solution of two liquids, Raoult’s law predicts that if no other gases are present, then the total pressure above the solution.
If a non-volatile solute is dissolved into a solvent to form an ideal solution, the vapour pressure of the final solution will be lower than that of the solvent. Hence, the decrease in the vapour pressure is directly proportional to the mole fraction of solute in an ideal solution. As water is having temperature around $65$ to $75$ around $30$ kPa thus the temperature of the water when the vapour pressure is $30$ kPa is $70{}^\circ C.$
Therefore, the correct answer is option C.

Note:
Remember that the temperature of the liquid increases, the kinetic energy also increases, the number of molecules transitioning into vapour also increases, and hence resulting in the increase in the vapour pressure.