Question

Which of the following solution having maximum vapour pressure at constant temperature $ ? $ [Molecular weight: $ NaCl = 58.5 $ ; $ {H_2}S{O_4} = 98.0 $ gram/mole]
(A) $ 1 $ molar $ {H_2}S{O_4} $
(B) $ 1 $ molal $ {H_2}S{O_4} $
(C) $ 1 $ molar $ NaCl $
(D) $ 1 $ molal $ NaCl $

Answer 1

Hint :First we know vapour pressure is the pressure of the vapour at equilibrium state when the rate of evaporation is equal to the rate of condensation. When comparing vapor pressures we need to be compared at the same temperature. Hence at room temperature, the substance with the lowest boiling point will have the highest vapor pressure and the substance with the highest boiling point will have the lowest vapor pressure.

Complete Step By Step Answer:
The vapour pressure of the solution increases as the concentration decreases. Because if a substance has high concentration, its boiling point is high. Example the vapour pressure of water decreases as the concentration of sucrose increases. Also, any solution has high vapour pressure if it loses a low number of ions.
Since one molar aqueous solution is more concentrated than one molal aqueous solution of the same solute.
In solution, $ {H_2}S{O_4} $ ​ provides three ions, while $ NaCl $ provides two ions. Hence, the vapour pressure of a solution of $ NaCl $ is higher (as it gives less number of ions).
Therefore, $ 1 $ molal $ NaCl $ will have the maximum vapour pressure.
Hence the correct option is D.

Note :
Molarity is the ratio of the moles of a solute to the total litres of a solution. The solution includes both the solute and the solvent. Molality is the ratio of the moles of a solute to the kilograms of a solvent. As the temperature of a liquid or solid increases (or decreases) its vapor pressure also increases (or decreases).