# Vapor Pressure of Electrolyte Solutions

## The vapor pressure of an electrolytic solution is dependent on the ratio of solute to solvent molecules in a solution.

#### Key Points

• Because vapor pressure is a colligative property, the vapor pressure of solutions is directly proportional the amount of solute present in a solution.

• When a solute is present in a solvent, the vapor pressure is lowered because less solvent molecules are present at the top of the solution. These spots are occupied by solute molecules and effectively make it harder for solvent to leave the solution.

• Raoult's Law details the calculations for acquiring the vapor pressure of an ideal solution. This equation shows the influence of each constituent on the vapor pressure of a solution.

#### Terms

• a substance that, in solution or when molten, ionizes and conducts electricity

• the pressure that one component of a mixture of gases would contribute to the total pressure

• In chemistry, an ideal solution or ideal mixture is a solution with thermodynamic properties analogous to those of a mixture of ideal gases.

#### Figures

1. ##### Effect of Solute on Vapor Pressure

The presence of solute lowers the vapor pressure of a volatile substance.

## Properties of Electrolyte Solutions

Most physical properties can be classified as intensive or extensive. Intensive properties are characteristic of the substance and do not depend on the size of the material being studied. Density is an example of an intensive property. Extensive properties are properties that are directly to the size of the material being studied. Mass is an example of an extensive property.

A third category of properties only applies to solutions—colligative properties. Properties can be considered colligative only if their properties are dependent on the amount of solute present in the solution, disregarding the identity of the solute itself. This property is most commonly seen in electrolyte solutions: those solutions that contain dissociated ions.

A simple example of an electrolyte solution is sodium chloride in water. In the presence of water, solid sodium chloride dissociates as it is dissolved, forming an electrolyte solution:

$NaCl_{(s)} \rightarrow Na^+_{(aq)} + Cl^-_{(aq)}$

Nonelectrolyte solutions are those in which the solute does not dissociate into ions (like sugar, for example) when dissolved. Electrolytes affect the colligative properties of solutions.

## Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its condensed phases (liquid or solid) at a particular temperature. Basically, it is a measure of how much the solvent molecules tend to escape from a liquid or solid phase into the atmosphere. Vapor pressure of a liquid is a colligative property. From an entropic point of view, evaporation is favorable because the entropy of a system will increase given that gaseous molecules occupy a larger volume than molecules in liquid form. However, if the original liquid solvent contains solute, the original entropy is larger, meaning the amount of entropy that can be gained upon liquid entering the gas phase is less. Therefore, there will be less solvent molecules entering the gas phase. This correlates to vapor pressure lowering as solute concentration increases.

(Figure 1)

To better visualize the effect of solute on the vapor pressure of a solution, consider a pure solvent. This pure solvent has a certain vapor pressure associated with it. Subjected to temperatures below the solvent's boiling point, the molecules changing to the gaseous phase are mostly situated on the top layer of the solution. Now consider a solution composed of both solvent and solute. Some solute molecules will thus occupy space near the surface of the liquid, decreasing the number of solvent molecules near the surface. Therefore, less molecules are changing from the liquid phase into the gas phase—the vapor pressure of the solvent has decreased.

In an ideal solution, the vapor pressure is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution. This is detailed by Raoult's law. In equilibrium, the total vapor pressure is $p = p_{A}^{\star}x_{A} + p_{B}^{\star}x_{B} + ...$

The individual vapor pressure for each component is $p_{i} = p_{i}^{\star}x_{i}$, where $p_{i}$ is the partial pressure of component i, $p_{i}^{\star}$ is the vapor pressure of pure component i and $x_{i}$ is the mole fraction of component i in the solution.

#### Key Term Glossary

atmosphere
a layer of gases that may surround a material body of sufficient mass, such as the earth, and that is held in place by the gravity of the body
##### Appears in these related concepts:
Boiling
Boiling is the rapid vaporization of a liquid and occurs when a liquid is heated to its boiling point. A liquid's boiling point is the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environment.
##### Appears in these related concepts:
boiling point
the temperature at which a liquid boils, with the vapor pressure equal to the given external pressure
##### Appears in these related concepts:
colligative properties
those that depend upon the ratio of the number of solute particles to the number of solvent molecules in a solution
##### Appears in these related concepts:
colligative property
The property of solutions that is directed by the ratio of solute to solvent, regardless of the identity of the solute.
##### Appears in these related concepts:
concentration
the proportion of a substance in a mixture
##### Appears in these related concepts:
density
a measure of the amount of matter contained by a given volume
##### Appears in these related concepts:
electrolyte
a substance that, in solution or when molten, ionizes and conducts electricity
##### Appears in these related concepts:
electrolytic
Of, relating to, or using electrolysis.
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entropy
A thermodynamic property that is the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work.
##### Appears in these related concepts:
equilibrium
the state of a reaction in which the rates of the forward and reverse reactions are the same
##### Appears in these related concepts:
Evaporation
Evaporation is a type of vaporization of a liquid that only occurs on the liquid's surface.
##### Appears in these related concepts:
extensive property
A property of matter that depends on the amount of matter that is being measured.
##### Appears in these related concepts:
fraction
a part of a whole, especially a comparatively small part
##### Appears in these related concepts:
gas
Matter in a state intermediate between liquid and plasma that can be contained only if it is fully surrounded by a solid (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly.
##### Appears in these related concepts:
ideal solution
In chemistry, an ideal solution or ideal mixture is a solution with thermodynamic properties analogous to those of a mixture of ideal gases.
##### Appears in these related concepts:
intensive property
A property of matter that does not depend on the amount of matter.
##### Appears in these related concepts:
ion
An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.
##### Appears in these related concepts:
liquid
A substance that flows and keeps no definite shape, such as water. A substance whose molecules, while not tending to separate from one another like those of a gas, readily change their relative position, and which therefore retains no definite shape, except that determined by the containing receptacle; an inelastic fluid.
##### Appears in these related concepts:
mass
The quantity of matter that a body contains, irrespective of its bulk or volume. It is one of four fundamental properties of matter. It is measured in kilograms in the SI system of measurement.
##### Appears in these related concepts:
mole
In the International System of Units, the base unit of the amount of substance; the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. Symbol: mol. The number of atoms in a mole is known as Avogadro’s number.
##### Appears in these related concepts:
molecule
the smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds
##### Appears in these related concepts:
mole fraction
The ratio of the number of moles of one component of a mixture to the total number of moles.
##### Appears in these related concepts:
nonelectrolyte
A substance that does not dissociate into ions when in solution, and therefore is a poor conductor of electricity when in solution
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partial pressure
the pressure that one component of a mixture of gases would contribute to the total pressure
##### Appears in these related concepts:
phase
Any one point or portion in a recurring series of changes, as in the changes of motion of one of the particles constituting a wave or vibration; one portion of a series of such changes, in distinction from a contrasted portion, as the portion on one side of a position of equilibrium, in contrast with that on the opposite side.
##### Appears in these related concepts:
physical property
A physical property is any property that is measurable whose value describes a physical system's state.
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Pressure
the amount of force that is applied over a given area divided by the size of this area
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ratio
The relative magnitudes of two quantities (usually expressed as a quotient).
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solid
A substance in the fundamental state of matter that retains its size and shape without need of a container (as opposed to a liquid or gas).
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solute
Any substance that is dissolved in a liquid solvent to create a solution.
##### Appears in these related concepts:
solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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solvent
a substance that dissolves a solute, resulting in a solution
##### Appears in these related concepts:
substance
Physical matter; material.
##### Appears in these related concepts:
system
the part of the universe being studied, arbitrarily defined to any size desired
##### Appears in these related concepts:
temperature
A measure of cold or heat, often measurable with a thermometer.
##### Appears in these related concepts:
vapor pressure
The pressure that a vapor exerts, or the partial pressure if it is mixed with other gases.
##### Appears in these related concepts:
volume
A unit of three-dimensional measure of space that comprises a length, a width, and a height. It is measured in units of cubic centimeters in metric, or cubic inches or cubic feet in English measurement.