# Vapor Pressure of Nonelectrolyte Solutions

## The vapor pressure of a solution is directly influenced by the number of solute molecules present in a given amount of solvent.

#### Key Points

• The vapor pressure of a system is a measure of how much the molecules tend to escape into the gas phase.

• Vapor pressure is a colligative property, meaning that the amount it is lowered or increased is directly related to the amount of solute present in the system.

• Raoult's law allows for the calculation of vapor pressure and shows the influence of each component in the total vapor pressure exerted by a system.

#### Terms

• The pressure that a vapor exerts, or the partial pressure if it is mixed with other gases.

• The property of solutions that is directed by the ratio of solute to solvent, regardless of the identity of the solute.

• a law stating that the vapour pressure of an ideal solution is directly dependent on the vapour pressure of each chemical component and the mole fraction of the component present in the solution

• those that depend upon the ratio of the number of solute particles to the number of solvent molecules in a solution

#### Figures

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

The vapor pressure of a solvent is lowered by the present of a solute.

Properties of chemical materials are generally classified as either chemical or physical. Chemical properties are associated with any characteristic associated with changing the material's identity; physical properties can be directly measured and often describe a material's state. For solutions, there exists a third class of properties known as colligative properties. These properties are solely dependent on the amount of solute in a given amount of solvent, and are unaffected by the identity of the solute. Colligative properties include freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure.

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature. It is a measure of the tendency of a material to escape into the environment via vapor, and is an indication of a liquid's evaporation rate. A substance that evaporates quickly has high vapor pressure and is referred to as a volatile substance. Introduction of solute to the solvent results in the lowering of the vapor pressure.

The effect of the presence of solute molecules on the vapor pressure can be understood from a free energy perspective. The tendency of molecules to escape into the gas phase is dependent on how much entropy can be gained by doing so. Escape into the gaseous phase of liquid molecules leads to a increase in entropy due to the greater volume occupied by the molecules in a gaseous state. If the liquid solvent becomes "diluted" with solute, the beginning entropy of the system is already larger, and the amount of entropy to be gained by having molecules escape into the gas phase will be lower. Therefore, one observes a decrease in vapor pressure. Imagine that, for a pure solvent, all the molecules that will escape into the gas phase are situated at the top of the liquid. Introduction of a solute to the solvent results in some of those top surface positions being occupied by solute instead of solvent molecules. Therefore, less solvent will leave the solution into the gas phase, correlating to a decrease in vapor pressure. The amount of solute is directly related to the amount that the vapor pressure is lowered by.(Figure 1)

Raoult's Law states that the vapor pressure of an ideal solution is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution. It is represented by:

$p = p^{\star}_{\rm A} x_{\rm A} + p^{\star}_{\rm B} x_{\rm B} + \cdots$

Each component's individual vapor pressure is denoted by:

$p_i = p^{\star}_i x_i$

where $p_i$ is the partial pressure of the component i in the solution, $p^{\star}_i$is the vapor pressure of the pure component i, and $x_i$ is the mole fraction of the component i in the solution. This law allows for the calculation of the vapor pressure of a given solution, and shows the influence of all of the molecules in the solution.

#### Key Term Glossary

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:
boiling point elevation
the phenomenon that the temperature at which a substance's vapor pressure equals the external pressure increases when another compound is added
##### Appears in these related concepts:
chemical property
any quality that can be determined only by changing a substance's molecular structure
##### 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:
energy
a quantity that denotes the ability to do work and is measured in a unit dimensioned in mass × distance²/time² (ML²/T²) or the equivalent
##### Appears in these related concepts:
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
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Evaporation
Evaporation is a type of vaporization of a liquid that only occurs on the liquid's surface.
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fraction
a part of a whole, especially a comparatively small part
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free energy
The difference between the internal energy of a system and the product of its entropy and absolute temperature.
##### Appears in these related concepts:
Freezing
Freezing or solidification is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point.
##### Appears in these related concepts:
freezing point
The temperature at which a liquid freezes, and the solid and liquid phases are in equilibrium; normally the same as the melting point.
##### Appears in these related concepts:
freezing point depression
the phenomenon that adding a solute to a solvent decreases the temperature at twhich the liquid solvent becomes a solid.
##### 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.
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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:
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.
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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
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mole fraction
The ratio of the number of moles of one component of a mixture to the total number of moles.
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osmotic pressure
the hydrostatic pressure exerted by a solution across a semipermeable membrane from a pure solvent; the pressure needed to counteract osmosis
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partial pressure
the pressure that one component of a mixture of gases would contribute to the total pressure
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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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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.
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solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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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
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state
The physical property of matter as solid, liquid, gas or plasma
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substance
Physical matter; material.
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system
the part of the universe being studied, arbitrarily defined to any size desired
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temperature
A measure of cold or heat, often measurable with a thermometer.
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thermodynamics
The science of the conversions between heat and other forms of energy.
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vapor pressure
The pressure that a vapor exerts, or the partial pressure if it is mixed with other gases.
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volatile
evaporating or vaporizing readily under normal conditions; having a low boiling point
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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.