Explanation:



What is Vapour Pressure?


Vapour pressure is the pressure exerted by the vapours over a liquid when the liquid and its vapour are in dynamic equilibrium. It is a measure of the tendency of a substance to evaporate and is dependent on temperature and intermolecular forces between the molecules of the liquid.



Raoult's Law:


Raoult's law states that the partial vapour pressure of each component of an ideal mixture of liquids is equal to the vapour pressure of the pure component multiplied by its mole fraction in the mixture.



Effect of solute on vapour pressure of the solvent:


When a non-volatile solute is added to a solvent, the vapour pressure of the solvent above the solution decreases. This is because the solute molecules occupy some of the surface area of the solvent and reduce the number of solvent molecules available to escape into the vapour phase. Therefore, the vapour pressure of the solution is lower than that of the pure solvent.



Calculating vapour pressure of an aqueous solution:


The vapour pressure of an aqueous solution can be calculated using the following formula:


P_solution = X_solvent * P_solvent


where P_solution is the vapour pressure of the solution, X_solvent is the mole fraction of the solvent in the solution and P_solvent is the vapour pressure of the pure solvent.



Solution to the given problem:


The given problem states that we have a 1 molal aqueous solution of solute at 100°C and we need to find the vapour pressure of the solution.


1 molal solution means that 1 mole of solute is dissolved in 1000 g of solvent (water) and the molality of the solution is 1 mol/kg.


From the given options, we can see that the vapour pressure of the pure solvent (water) at 100°C is 1 atm (option A).


Now, we need to find the mole fraction of water in the solution to calculate the vapour pressure of the solution using Raoult's law.


Mole fraction of water = moles of water / total moles of solute and solvent


Since we have a 1 molal solution, the mass of solvent (water) is 1000 g and its molar mass is 18 g/mol. Therefore, the number of moles of water in the solution is:


moles of water = 1000 g / 18 g/mol = 55.56 mol


The number of moles of solute is also 1 mol since we have a 1 molal solution.


Total moles of solute and solvent = 55.56 + 1 = 56.56 mol


Mole fraction of water = 55.56 / 56.56 = 0.9823


Using Raoult's law, we can