Vapour Pressure

Vapour Pressure of Solution

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:35 PM IST

Francois Marie Raoult discovered the vapour of solution in the end of the 19th century. And his work was published in the years of 1887 to 1888 by which everyone got to know about Raoult's law. François-Marie Raoult, was a french chemist and studied about the vapour pressure. He observed that when a non-volatile solute is added to the solvent the vapour of the solution changes. This is what he states in his discovery which is Raoult's law in this law he states that the when vapour pressure of solvent in the ideal solution is directly proportional to the mole fraction of the solvent in the solution.

This Story also Contains

Vapour Pressure
Factors On Which Vapour Pressure Depends
Some Solved Examples
Summary

Vapour Pressure of Solution

He experimented on water and benzene and measured their vapour pressure by adding different non-volatile solutions to them. In performing this series of experiments he demonstrated that the addition of a non-volatile solute decreased the vapour pressure of the solvent, which he aligned with his law. Raoult's law is a Solutions chemistry framework and has various applications in the scientific and industrial fields. He provides the quantitative method For knowing the behaviour of the solution, which is very sensitive in drug formulations, manufacturing of chemicals and in the field of material science.

It is the pressure exerted by vapours of a pure liquid over its surface when they are in equilibrium with the liquid at a given temperature.

For example, if we take the case of water, then the equilibrium constant of the following physical process will represent the Vapor Pressure of Water (Also sometimes called as Aqueous Tension)

H2O(l))⇌H2O(g),Kp=Po

At equilibrium, the rate of vaporisation = the rate of condensation and the equilibrium constant of the above vapour-liquid equilibrium represents the vapour pressure of the liquid.

It depends upon the nature of the liquid and temperature. Pure liquid has always a vapour pressure greater than its solution.
The vapour pressure of a liquid helps us to have an idea of forces of attraction amongst the molecules of liquid that is, the more the force of attraction, the lower the vapour pressure and vice versa.
The vapour pressure of a liquid increases with an increase in temperature due to an increase in the kinetic energy of solvent molecules that is, an increase in evaporation however it is independent of the nature of the vessel.

Vapour Pressure of a Solution
When a miscible solute is added to a pure solvent, it results in the formation of a solution. As some molecules of solute will replace the molecules of the solvent from the surface, therefore, escaping tendency of solvent molecules decreases. This causes a lowering of vapour pressure.

The vapour pressure of a solution is less than that of the pure solvent.
If the vapour pressure of the solvent is P and that of the solution is P_s then lowering of V.P = P - P_s.
The vapour pressure of the solution decreases as the surface area occupied by the solvent molecule decreases and density increases.

Factors On Which Vapour Pressure Depends

Factors governing the vapour pressure:

1. Temperature: As Temperature increases, the Kinetic Energy of the molecules in the liquid phase increases and as a result, more molecules are able to escape to the gaseous phase and hence the vapour Pressure increases.

The Clausius Clapeyron Equation relates the vapour pressure of the liquid to the temperature and is given as

ln⁡(P2P1)=ΔHR(1 T1−1 T2)

Where ΔH is the heat of vaporisation of the liquid and P1 and P2 are the vapour pressure at temperature T₁ and T₂ respectively.

2. Vapour pressure depends on the nature of the liquid. The greater the force of attraction between the liquid molecules, the lesser is the vapour pressure

3. vapour pressure does not depend on the shape or the size of the container and has a fixed value at a particular temperature.

Significance of vapour pressure:

Vapour pressure gives us an idea of intermolecular forces of attraction in the liquid. The greater the force of attraction, the lower is the vapour pressure and vice versa.

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Vapour pressure gives us an idea of the volatility (vapour-forming tendency of the liquid). The greater the vapour pressure, the greater is the volatility of the liquid.
Vapour pressure also gives an idea of the boiling point of the liquid. The greater the vapour pressure, the lesser is the boiling point of the liquid.

Some Solved Examples

Example.1

1. When a substance is dissolved in a solvent the vapour pressure of the solvent is decreased. This results in:

1) (correct)An increase in the b.p. of the solution

2)A decrease in the b.p. of the solvent

3)The solution has a higher freezing point than the solvent

4)The solution has a lower osmotic pressure than the solvent

Solution

Vapour pressure α1 Boiling point

When vapour pressure decreases then b.pt. increases.
Hence, the answer is the option (1).

Example.2

2. An aqueous solution of methanol in water has vapour pressure:

1)Equal to that of water

2)Less than that of water

3) (correct)More than that of water

4)We can't say

Solution

Methanol has a lower boiling point than H₂O.
The lower is the boiling point of the solvent more is vapour pressure.

Hence, the answer is the option (3).

Example.3

3. Which has the maximum vapour pressure?

1)HI

2)HBr

3) (correct)HCl

4)HF

Solution

The lower the boiling point the greater is vapour pressure.
boiling point order: HCl < HBr < HI < HF
Hence, the answer is the option (3).

Example.4

4. A mixture of 100 m mol of Ca(OH)2 and 2 g of sodium sulphate was dissolved in water and the volume was made up to 100 mL.The mass of calcium sulphate formed and the concentration of $OH^{-}$ in the resulting solution, respectively , are : (Molar mass of Ca(OH)2,Na2SO4 and CaSO4 , are 74, 143 and 136 g mol−1 , respectively ; Ksp of Ca(OH)2 is 5.5×10−6,)

1) (correct)1.9 g,0.28 mol L−1

2)13.6 g,0.28molL−1

3)1.9 g,0.14molL−1

4)13.6 g⋅0.14 mol−1

Solution

Given,

Mol of Na₂SO₄ = 2/142 = 14 m mol

Ca(OH)2+Na2SO4⟶CaSO4+2NaOHmmol1001414 m/mol28 m/mol

Mass of CaSO4=14×1361000=1.9gm
Molarity of OH−=28100=0.28 mol/L

Example.5

5. Which of the following has the highest vapour Pressure at Room temperature?

1)Benzene

2)Toluene

3)n-Hexane

4) (correct)Naphthalene

Solution

Vapour Pressure α Surface area

α Molecular Mass

Among the given compounds naphthalene has a higher molecular Mass.

Hence, the answer is the option (4).

Example.6

6. Three solutions are prepared by adding ‘w’ g of ‘A’ into 1 kg of water, ‘w’ g of ‘B’ into another 1 kg of water and ‘w’ g of ‘C’ in another 1 kg of water (A, B, C are non-electrolytic). Dry air is passed from these solutions in sequence (A $\rightarrow$ B $\rightarrow$ C). The loss in weight of solution A was found to be 2 g while solution B gained 0.5 g and solution C lost 1 g. Then the relation between molar masses of A,B and C is :

1)MA:MB:MC=4:3:5

2)MA:MB:MC=14:13:15

3) (correct)MC>MA>MB

4),MB>MA>MC

Solution

The loss in weight from A is due to dry air and proportional to vapour pressure above that solution. The loss or gain in mass from solution B or C will depend upon the Pressure above that solution and the pressure inside the solution from which it is coming into the particular solution.

So,

PA∝2gm

(PA−PB)∝0.5gm(PC−PB)∝2.5gm

Hence, the order of Pressure: P_C > P_A> P_B

So, the maximum vapour pressure is above solution C hence, it has minimum lowering and hence the minimum mole fraction (hence a minimum number of moles) of solute So, it has a maximum molar mass of the substance.

Hence, the answer is the option (3).

Summary

The vapour pressure of a solution is a fundamental concept in chemistry which tells the capability of liquid to change in the gaseous phase means that vapour pressure is the pressure in which the liquid can change into the gaseous state. This property is essential for understanding various phenomena and processes in both theoretical and practical fields. Vapour pressure has a very important role in predicting the boiling point of any solution, which is the temperature at which its vapour pressure equals the atmospheric pressure. As a result, solutions with lower vapour pressures generally have higher boiling points compared to their pure solvents.

Frequently Asked Questions (FAQs)

1. How does adding a solute affect the vapor pressure of a solution?

Adding a solute generally decreases the vapor pressure of a solution compared to the pure solvent. This is because the solute particles interfere with the evaporation of solvent molecules, reducing the number of solvent molecules that can escape into the vapor phase.

2. Why do real solutions often deviate from Raoult's Law?

Real solutions often deviate from Raoult's Law due to interactions between solute and solvent molecules. These interactions can be attractive (causing negative deviations) or repulsive (causing positive deviations), leading to vapor pressures that are lower or higher than predicted by Raoult's Law.

3. What is vapor pressure lowering?

Vapor pressure lowering is the decrease in vapor pressure of a solution compared to the pure solvent. It occurs when a non-volatile solute is added to a volatile solvent, reducing the number of solvent molecules available at the surface to evaporate.

4. How does temperature affect the vapor pressure of a solution?

Increasing temperature generally increases the vapor pressure of a solution. This is because higher temperatures provide more kinetic energy to the molecules, allowing more of them to overcome intermolecular forces and escape into the vapor phase.

5. What is Raoult's Law?

Raoult's Law states that the partial vapor pressure of each component in an ideal solution is directly proportional to its mole fraction in the solution. Mathematically, it's expressed as P = X * P°, where P is the partial vapor pressure, X is the mole fraction, and P° is the vapor pressure of the pure component.

6. What is vapor pressure?

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It represents the tendency of particles to escape from the liquid or solid phase into the gas phase.

7. What is the relationship between vapor pressure and boiling point?

The boiling point of a liquid is the temperature at which its vapor pressure equals the atmospheric pressure. As vapor pressure increases with temperature, the boiling point is reached when the vapor pressure matches the surrounding pressure.

8. What is an ideal solution in terms of vapor pressure?

An ideal solution is one that obeys Raoult's Law exactly. In an ideal solution, the interactions between solute and solvent molecules are similar to those between molecules of the pure components, resulting in no heat of mixing and no volume change upon mixing.

9. How does the volatility of components affect the vapor pressure of a solution?

In a solution of two or more volatile components, each component contributes to the total vapor pressure based on its mole fraction and its pure vapor pressure. More volatile components (those with higher pure vapor pressures) will contribute more to the total vapor pressure of the solution.

10. What is a colligative property, and how is vapor pressure related to it?

A colligative property is a characteristic of a solution that depends on the number of dissolved particles, not their nature. Vapor pressure lowering is a colligative property because it depends on the concentration of solute particles, regardless of their chemical identity.

11. What is Henry's Law?

Henry's Law states that the amount of dissolved gas in a liquid is directly proportional to the partial pressure of the gas above the liquid, at a constant temperature. It's expressed as C = k * P, where C is the concentration of dissolved gas, k is Henry's constant, and P is the partial pressure of the gas.

12. How does Henry's Law relate to the solubility of gases in liquids?

Henry's Law describes how the solubility of gases in liquids changes with pressure. As the partial pressure of a gas above a liquid increases, more gas molecules dissolve in the liquid, increasing its concentration in solution.

13. How does intermolecular force strength affect vapor pressure?

Stronger intermolecular forces generally lead to lower vapor pressures. This is because molecules with stronger attractions require more energy to overcome these forces and escape into the vapor phase.

14. How does the vapor pressure of a solution relate to its osmotic pressure?

Both vapor pressure lowering and osmotic pressure are colligative properties related to the concentration of solute particles. The van 't Hoff factor, which accounts for dissociation or association of solute particles, affects both properties similarly.

15. What is the effect of an electrolyte solute on vapor pressure lowering?

Electrolyte solutes generally cause a greater vapor pressure lowering than non-electrolytes at the same molal concentration. This is because electrolytes dissociate into multiple ions, effectively increasing the number of particles in solution and enhancing the colligative effect.

16. How does the molecular weight of a solute affect vapor pressure lowering?

For a given mass of solute, a lower molecular weight results in a greater number of particles in solution, leading to a larger vapor pressure lowering effect. This is because colligative properties depend on the number of particles, not their mass.

17. What is the significance of Dalton's Law of Partial Pressures in understanding vapor pressure of solutions?

Dalton's Law states that the total pressure of a mixture of gases is the sum of the partial pressures of each gas. This principle applies to the vapor pressure of solutions with multiple volatile components, where each component contributes its partial pressure to the total vapor pressure.

18. How does azeotrope formation relate to vapor pressure in solutions?

An azeotrope is a mixture of two or more liquids whose proportions cannot be altered by simple distillation. This occurs when the vapor pressure curve of the mixture reaches a maximum or minimum point, where the composition of the vapor is the same as that of the liquid.

19. What is the vapor pressure diagram, and how is it used?

A vapor pressure diagram is a graph showing the relationship between vapor pressure and composition for a binary mixture. It's used to visualize how vapor pressure changes with composition and to identify azeotropes or deviations from ideal behavior.

20. How does the presence of a surfactant affect the vapor pressure of a solution?

Surfactants can lower the surface tension of a liquid, which can slightly increase its vapor pressure. However, the effect is usually small compared to the vapor pressure lowering caused by dissolved solutes.

21. What is the relationship between vapor pressure and evaporation rate?

The vapor pressure of a liquid is directly related to its evaporation rate. A higher vapor pressure indicates a greater tendency for molecules to escape into the gas phase, resulting in a faster evaporation rate.

22. How does atmospheric pressure affect the observed vapor pressure of a solution?

Atmospheric pressure doesn't directly affect the vapor pressure of a solution, which is an intrinsic property. However, it affects the boiling point, which is the temperature at which the vapor pressure equals the atmospheric pressure.

23. What is the effect of adding a second volatile component to a solution on its total vapor pressure?

Adding a second volatile component generally increases the total vapor pressure of the solution. The total vapor pressure becomes the sum of the partial pressures of both components, each contributing according to its mole fraction and pure vapor pressure.

24. How does the concept of fugacity relate to vapor pressure in non-ideal solutions?

Fugacity is a measure of the tendency of a substance to escape from a phase, similar to vapor pressure but accounting for non-ideal behavior. In non-ideal solutions, fugacity is used instead of vapor pressure to more accurately describe the escaping tendency of molecules.

25. What is the significance of vapor pressure in distillation processes?

Vapor pressure differences between components in a mixture are the basis for distillation. Components with higher vapor pressures will vaporize more readily, allowing for separation based on differences in boiling points and vapor compositions.

26. How does the presence of dissolved gases affect the vapor pressure of a liquid?

Dissolved gases can increase the total vapor pressure above a liquid. The contribution of the dissolved gas to the vapor pressure is described by Henry's Law, while the solvent's contribution follows Raoult's Law.

27. What is the relationship between vapor pressure and the rate of condensation?

At equilibrium, the rate of evaporation equals the rate of condensation. The vapor pressure represents this equilibrium state. A higher vapor pressure indicates both a higher rate of evaporation and a higher rate of condensation at equilibrium.

28. How does the shape of molecules affect the vapor pressure of a liquid?

Molecular shape affects intermolecular forces, which in turn influence vapor pressure. For example, linear molecules often have stronger intermolecular attractions and lower vapor pressures compared to branched isomers of the same molecular formula.

29. What is the effect of pressure on the vapor pressure of a solution?

External pressure has minimal direct effect on vapor pressure, which is an intensive property of the liquid. However, very high pressures can slightly increase vapor pressure by compressing the liquid and altering intermolecular distances.

30. How does the vapor pressure of a solution relate to its chemical potential?

The chemical potential of a component in solution is related to its vapor pressure. In an ideal solution, the difference in chemical potential between the solution and pure component is proportional to the natural logarithm of the ratio of vapor pressures.

31. What is the significance of Clausius-Clapeyron equation in understanding vapor pressure?

The Clausius-Clapeyron equation describes how vapor pressure changes with temperature. It relates the slope of the vapor pressure curve to the enthalpy of vaporization and is useful for predicting vapor pressures at different temperatures.

32. How does the presence of a non-volatile solute affect the vapor-liquid equilibrium of a solution?

A non-volatile solute shifts the vapor-liquid equilibrium by lowering the vapor pressure of the solution. This results in a higher boiling point and a lower freezing point compared to the pure solvent.

33. What is the role of vapor pressure in the formation of clouds and precipitation?

Vapor pressure plays a crucial role in atmospheric processes. When the partial pressure of water vapor in air exceeds the saturation vapor pressure at a given temperature, condensation occurs, leading to cloud formation and potentially precipitation.

34. How does the concept of activity coefficient relate to vapor pressure in non-ideal solutions?

The activity coefficient is a factor that accounts for deviations from ideal behavior in solutions. It modifies Raoult's Law for non-ideal solutions, relating the actual vapor pressure to the ideal vapor pressure predicted by Raoult's Law.

35. What is the relationship between vapor pressure and the triple point of a substance?

The triple point is the unique temperature and pressure at which solid, liquid, and vapor phases of a substance coexist in equilibrium. The vapor pressure at the triple point is equal to the pressure of the triple point itself.

36. How does the presence of an ionic solute affect the vapor pressure of water in a solution?

Ionic solutes generally cause a greater reduction in vapor pressure than non-ionic solutes at the same concentration. This is due to the dissociation of ionic compounds, which increases the effective number of particles in solution.

37. What is the significance of vapor pressure in understanding the concept of volatility?

Vapor pressure is a direct measure of a substance's volatility. Substances with higher vapor pressures are more volatile, meaning they evaporate more readily at a given temperature.

38. How does the vapor pressure of a solution relate to its colligative properties?

Vapor pressure lowering is itself a colligative property and is directly related to other colligative properties such as boiling point elevation, freezing point depression, and osmotic pressure. All these properties depend on the concentration of solute particles.

39. What is the effect of a mixed solvent system on vapor pressure?

In a mixed solvent system, each solvent contributes to the total vapor pressure based on its mole fraction and pure vapor pressure. The resulting vapor pressure can be higher or lower than that of either pure solvent, depending on the specific interactions between the solvents.

40. How does the concept of partial molar volume relate to vapor pressure in solutions?

Partial molar volume, which describes how the total volume of a solution changes with the addition of a component, can affect solution non-ideality. This non-ideality can lead to deviations from Raoult's Law and affect the vapor pressure of the solution.

41. What is the relationship between vapor pressure and the critical point of a substance?

As a substance approaches its critical point, the distinction between liquid and vapor phases becomes less defined. At the critical point, the vapor pressure becomes equal to the critical pressure, above which the substance cannot exist as a liquid regardless of pressure.

42. How does the presence of a polymer in solution affect vapor pressure?

Polymers in solution can significantly lower the vapor pressure due to their large molecular size. Even at low mass concentrations, polymers can have a substantial effect on colligative properties, including vapor pressure lowering.

43. What is the significance of vapor pressure in understanding the concept of vapor-liquid equilibrium (VLE)?

Vapor pressure is fundamental to VLE. At equilibrium, the rate of evaporation equals the rate of condensation, and the pressure exerted by the vapor is the vapor pressure. VLE diagrams use vapor pressure data to show the compositions of coexisting liquid and vapor phases.

44. How does the vapor pressure of a solution relate to its enthalpy of vaporization?

The enthalpy of vaporization is related to vapor pressure through the Clausius-Clapeyron equation. Substances with higher enthalpies of vaporization generally have lower vapor pressures at a given temperature, as more energy is required for vaporization.

45. What is the effect of surface curvature on vapor pressure, particularly for small droplets?

The Kelvin equation describes how vapor pressure increases for very small droplets due to surface curvature. This effect, known as the Kelvin effect, is significant for droplets on the nanometer scale and plays a role in cloud formation and stability of emulsions.

46. How does the concept of escaping tendency relate to vapor pressure?

Escaping tendency is a qualitative concept that describes the propensity of molecules to leave a phase. Vapor pressure is a quantitative measure of this escaping tendency for liquids, representing the pressure exerted when the tendency to escape is balanced by the tendency to return to the liquid phase.

47. What is the relationship between vapor pressure and the rate of effusion in gases?

While vapor pressure itself doesn't directly determine effusion rates, both are related to molecular motion and intermolecular forces. Gases with higher vapor pressures at a given temperature generally have higher rates of effusion due to the greater kinetic energy of their molecules.

48. How does the presence of dissolved salts affect the vapor pressure of seawater compared to pure water?

Dissolved salts in seawater lower its vapor pressure compared to pure water. This vapor pressure lowering is a colligative property and depends on the total concentration of dissolved ions. As a result, seawater has a slightly higher boiling point than pure water.

49. What is the significance of vapor pressure in understanding the process of fractional distillation?

Fractional distillation relies on differences in vapor pressures between components of a mixture. Components with higher vapor pressures will vaporize at lower temperatures and can be separated based on their different boiling points, which are directly related to their vapor pressures.

50. How does the concept of activity relate to vapor pressure in non-ideal solutions?

Activity is a measure of the "effective concentration" of a species in a non-ideal mixture. In the context of vapor pressure, the activity of a component in solution replaces its mole fraction in Raoult's Law for non-ideal solutions, providing a more accurate description of the component's contribution to the total vapor pressure.