Which one of these properties is directly proportional to molality and why?
A) Electrical conductance
B) Heat of solution
C) Equilibrium of solution
D) Vapour pressure
Thanks
To determine which property is directly proportional to molality, we need to understand the concept of molality and its relationships with other properties.
Molality (denoted as "m") is a measure of the concentration of a solution expressed in terms of the number of moles of solute (the substance being dissolved) per kilogram of solvent. Unlike molarity, which uses the volume of the solvent, molality uses the mass of the solvent, making it a more reliable measure for studying properties that depend on the number of particles rather than the volume.
Now, let's examine each property and its relationship with molality:
A) Electrical conductance: Electrical conductance measures the ability of a solution to conduct electricity. In general, the electrical conductance of a solution depends on the number of ions present, which in turn is determined by the concentration of solute particles in the solvent. However, molality alone does not provide information about the dissociation of solute into ions. So, it is not directly proportional to molality.
B) Heat of solution: The heat of solution is the amount of heat energy absorbed or released when a solute dissolves in a solvent. The heat of solution depends on several factors, including the nature of the solute and solvent and the strength of their interactions. While the concentration of the solute, which can be expressed in molality, can affect the heat of solution, it is not directly proportional to molality.
C) Equilibrium of solution: The equilibrium of a solution is determined by various factors, such as temperature, pressure, and the nature of the solute and solvent. Equilibrium is not directly dependent on molality. Therefore, it is not directly proportional to molality.
D) Vapour pressure: Vapour pressure is the pressure exerted by the vapor of a liquid when it is in equilibrium with its pure liquid or solid state. Molality affects the vapor pressure of a solution through a property called Raoult's law, which states that the partial pressure of each solvent component above the solution is directly proportional to the mole fraction of that component. Since mole fraction is closely related to molality, vapour pressure is directly proportional to molality.
Therefore, among the given properties, the one directly proportional to molality is D) Vapour pressure.