Vapour pressure is the force exerted by a liquid’s (or solid’s) vapour above the surface of the liquid. At a particular temperature and thermodynamic equilibrium, this pressure is formed in a closed container. The rate of liquid evaporation is controlled by the equilibrium vapour pressure. The vapour pressure increases with increasing temperature. When atmospheric pressure and vapour pressure are equal, a liquid is said to have reached its boiling point.

Vapour Pressure-Definition, Raoult’s Law, Formula, Factors Affecting Vapour Pressure & FAQs

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Vapour pressure is the force exerted by a liquid’s (or solid’s) vapour above the surface of the liquid. At a particular temperature and thermodynamic equilibrium, this pressure is formed in a closed container. The rate of liquid evaporation is controlled by the equilibrium vapour pressure. The vapour pressure increases with increasing temperature. When atmospheric pressure and vapour pressure are equal, a liquid is said to have reached its boiling point.

Characteristics of Vapour Pressure

Vapour pressure is defined as the pressure exerted by the liquid in the thermodynamic equilibrium state. Any substance that has a high vapour pressure at normal temperature is termed a volatile substance. The vapour pressure at the equilibrium state indicates the rate of evaporation in the liquid.

In the stage of equilibrium, the pressure exerted by the atmosphere is equal to the vapour of the liquid.

Factors Affecting Vapour Pressure

Vapour Pressure of the liquid depends on the various factor discussed below:

  • Nature of Liquid: Intermolecular forces between the atoms of the liquid explain the nature of the liquid. Vapour Pressure of the liquid changes according to different types of liquids.
  • Effect of Temperature: With the increase in temperature of the liquid, the kinetic energy of the atoms of the liquid also increases. Now, the liquid molecules with high kinetic energy are less likely to escape thus, the vapour pressure of the liquid increases. Hence we can say that vapour pressure is directly proportional to temperature.
  • Concentration of Solute: The existence of a solute in the liquid will significantly reduce the vapour pressure. And this fall in vapour pressure also differs with respect to the concentration of solute.

What is Raoult’s Law?

According to Raoult’s law, the vapour pressure of a pure component (liquid or solid) multiplied by its mole fraction in the mixture results in the partial pressure of that component in a perfect mixture of liquids. As a result, the mole fraction of solute in the solution equals the relative decrease in vapour pressure of a diluted solution of a non-volatile solute.
A solution that complies with Raoult’s law is referred to as the “perfect solution”. While some liquid combinations adhere to this law over a wide range of concentrations, this law only holds for diluted solutions. Perfect solutions are produced because the intermolecular forces between molecules of pure substances are the same as the forces between molecules of one material and molecules of another.

Raoult’s Law or Vapour Pressure Formula

When a solid is dissolved into a liquid, a solution is made. When the solute is added, the vapour pressure that results from this solution is reduced. Here is the formula for vapour pressure using Raoult’s law, which explains how the vapour pressure of a liquid changes when a solute is added.

Psolution = (Xsolvent)×(P°solvent)

where,

Psolution is the vapour pressure of a solution
Xsolvent is the mole fraction of solvent in a solution
P°solvent is the vapour pressure of a solvent

Importance of Raoult’s law

Let’s say a volatile liquid A is placed inside a closed container. After some time, vapour particles will start to develop as a result of evaporation. The liquid particles on the surface will eventually be in dynamic equilibrium with the vapour particles of A as time goes on. As a result, the pressure that A’s vapour particles are exerting at any given temperature is known as A’s vapour pressure at that temperature. All solids and liquids display vapour pressure, which is solely dependent on the kind of liquid and temperature.

Vapour pressure is the force exerted by a liquid’s (or solid’s) vapour above the surface of the liquid. At a particular temperature and thermodynamic equilibrium, this pressure is formed in a closed container. The rate of liquid evaporation is controlled by the equilibrium vapour pressure. The vapour pressure increases with increasing temperature. When atmospheric pressure and vapour pressure are equal, a liquid is said to have reached its boiling point.

The A particles will now fill the spaces between the B particles on the surface of the solution if liquid B is introduced to this container.
A portion of the molecules on the surface of any given liquid have enough energy to convert to the vapour phase.
Since there are currently fewer A particles on the surface, there will be fewer A vapour particles in the vapour phase. This will cause A’s vapour pressure to decrease.
If B is also volatile, there will be fewer B particles in the vapour phase than there would be if B were a pure liquid.

Vapour pressure is the force exerted by a liquid’s (or solid’s) vapour above the surface of the liquid. At a particular temperature and thermodynamic equilibrium, this pressure is formed in a closed container. The rate of liquid evaporation is controlled by the equilibrium vapour pressure. The vapour pressure increases with increasing temperature. When atmospheric pressure and vapour pressure are equal, a liquid is said to have reached its boiling point.

This new pressure, which is determined by Raoult’s equation and depends on the component concentration of each liquid phase, is the partial pressure of each A and B

PA ∝ XA = XAP°A 

PB ∝ XB  = XBP°B

P° represents the component of the mole fraction.

Vapour Pressure of Liquid-Liquid Solutions

Let’s use two volatile liquid solutions labelled 1 and 2 to calculate the vapour pressure of a liquid-liquid solution. An equilibrium between the binary solution’s liquid phase and vapour phase is created when the liquid components are added to a closed vessel.
Assume that PTotal is the total vapour pressure of the binary solution at equilibrium and P1 and P2 are the respective partial vapour pressures of components 1 and 2. Let the mole fractions of components 1 and 2 be X1 and X2, as the partial pressures are related to the mole fractions.

Vapour Pressure of Solutions of Solids in Liquids

We mix sugar and water to create a sugar solution. The non-volatile solute in this mixture is sugar, while the solvent is water. Due to the non-volatility of the solute, when this sugar solution evaporates, the vapour phase only consists of water vapours. However, it is found that this solution’s vapour pressure is lower than that of the pure solvent (water).

The rate at which the solvent molecules escape from the surface of the liquid determines the vapour pressure of a solution made up of a non-volatile solute and a solvent. As a result, the vapour pressure of the solution (sugar and water) will be lower than that of the pure solvent (water), or the addition of a non-volatile solute will cause the vapour pressure of the solution to decrease. The solution’s vapour pressure will decrease more when the concentration of sugar in the solution rises.
The amount of non-volatile solute present in the solution, not the type of non-volatile solute, determines how much of the solvent’s vapour pressure is reduced.

What is a boiling point?

Vapour pressure of any liquid increases with the increase in temperature as the temperature increases there comes a stage at which the vapour pressure of the liquid is equal to the atmospheric pressure. It reaches a stage where the vapour pressure of the liquid becomes equal to the atmospheric pressure. In this stage, the vapour of liquid escapes to the atmosphere and the temperature of the liquid at this stage is called the boiling point of the liquid.

For the standard value of the boiling point of liquid pressure is taken to be,

Pressure = 1 atm = 102325 Pa = 1 bar

What is the Heat of Vapourization?

The heat required by 1 mole of liquid to change its phase from liquid to vapour form is called the Heat of Vapourization. The heat of Vapourization only changes the phase of the liquid to gaseous form it does not increase the temperature of the liquid.

Raoult’s Law and its Relationship with Other Laws

Raoult’s Law is one of the basic laws and its relation with other laws can be stated as,

  • Ideal gas law and Raoult’s law resemble one other quite a bit. Raoult’s law does not apply to solutions, which is the single exception. If you’ve read about the ideal gas law, you know that it implies that gases would behave in an ideal manner, with zero or no intermolecular forces between molecules of different types. Meanwhile, Raoult’s law makes the hypothesis that the intermolecular forces between unlike and comparable molecules are equal.
  • Non-ideal solutions are similarly subject to Raoult’s law. The interactions between molecules of various substances must be taken into account when incorporating several components, though.
    If we use an ideal system made up of an ideal liquid and an ideal vapour, we may also deduce a very
  • By combining Raoult’s law and Dalton’s law, we may further obtain a highly helpful equation for an ideal system made up of an ideal liquid and an ideal vapour.
  • This equation tells us that each component of an ideal solution made up of pure substances will have a different vapour pressure. Additionally, the component will have a higher pure vapour pressure in the gas phase compared to the solution, which will have a lower pure vapour pressure.

Limitations of Raoult’s Law

Raoult’s Law has limitations which are discussed below,

  • Raoult’s law is especially relevant since it refers to ideal solutions, i.e., those in which the gas phase has thermodynamic properties similar to a combination of ideal gases. 
  • The only drawback is that they are hard to find and rare. Different chemical components must be chemically equivalent. 
  • Several solutions deviate from Raoult’s law because various attractive forces exist in many liquid mixtures. So, do not properly obey it.

Solved Examples on Vapour Pressure Formula

Example 1: At 25 °C, an aqueous solution’s vapour pressure is measured to be 23.80 mmHg. What fraction of the solute by a mole in this solution? At 25 °C, water has a vapour pressure of 25.756 mm Hg.

Solution:

Vapour pressure of the solution, P Solution = 23.80

Vapour pressure of solvent, P° Solvent= 25.756

Using vapour pressure formula:

Psolution = (Xsolvent) (P°solvent)

23.80= (XSolvent) 25.756

XSolvent = 0.92405

Mole fraction of Solvent = 0.92405

Mole fraction of Solute = 1 – 0.92405

                                      = 0.07595

Example 2: At 25°C, an aqueous solution’s vapour pressure is measured at being 20mmHg. What is the mole fraction of the solvent in this solution? At 25 °C, water has a vapour pressure of 60 mm Hg.

Solution: 

Vapour pressure of the solution, P Solution = 20

Vapour pressure of solvent, P° Solvent= 60

Using vapour pressure formula:

Psolution = (Xsolvent) (Posolvent)

20= (XSolvent) 60

XSolvent = 0.333

Mole fraction of solvent = 0.333

Example 3: What is the vapour pressure of the solution if the mole fraction of the solute is 0.3? Water has a 16.358 mmHg vapour pressure at 23 °C.

Solution:

Mole Fraction of solute = 0.3

Mole Fraction of solvent, XSolvent = 1 – 0.3

                                       = 0.7

Now,

Vapour pressure of solvent, P°Solvent = 16.358

Mole Fraction of solvent = 0.7

Using vapour pressure formula:

Psolution = (Xsolvent) (P°solvent)

            = (0.7)(16.358)

            = 11.4506

Vapour pressure of the solution is 11.4506

Example 4: 200 grams of sucrose dissolved in 300 grams of water at 30 degrees. How much vapour pressure does this solution have? At 30.0 °C, the vapour pressure of water is 41.62 mmHg.

Solution: 

First, we need to find the fraction of mole in a solvent.

Now, the amount of mole in solute

Moles of Sucrose = wt (gm)/ molecular mass              (molecular weight of sucrose = 342.2948)
                             = 200 / 342.2948g
                             = 0.5843 

Moles of Water = 300 / 18.015                                     (molecular weight of water = 18.015)
                          = 16.6528 

Total mole of solution = 0.5843+16.6528 = 17.2371

Mole fraction of solvent (water) = Mole of water / Mole of solution

Mole fraction of solvent, X Solvent = 16.6528/(16.6528 + 0.5843)

                                                     = 16.6528/17.2371

                                                     = 0.9661

vapour pressure of solvent, P°Solvent = 41.62

Using vapour pressure formula:

Psolution = (Xsolvent) (P°solvent)

           = (0.9661)(41.62)

           = 40.2090

Vapour pressure of the solution is 40.2090.

Example 5: What is the vapour pressure of 400 g of propanol (molecular weight = 60 g/mol) and 130 g of acetone (molecular weight = 58 g/mol)? Propanol and acetone have vapour pressures of 11 and 20 mmHg, respectively, at 25 °C.

Solution:

CalculatingMolar fractions: 

Number of moles in acetone, nacetone = 130g / 58g/mol = 2.24 mol

Number of moles in propanol, npropanol = 400g / 60g/mol = 6.66 mol

ntotal = nacetone + npropanol = 2.24 + 6.66 = 8.9 mol

Molar fraction of acetone, xacetone= 2.24/8.9 = 0.2516

Molar fraction of propanol, xpropanol = 6.66/8.9 = 0.7483

Vapour pressure of acetone, Pacetone = 20

Vapour pressure of Propanol, P°Propanol= 11

Now, partial pressure can be calculated as:

Pacetone = (xacetone) ( P°acetone) = 0.2516 × 20 = 5.03 mmHg

PPropanol = (xPropanol) ( P°Propanol) = 0.7483 × 11 = 8.2313 mmHg

vapour Pressure of the Mixture

Pmixture = Pacetone + PPropanol 
            = 5.03 + 8.2313 
            = 13.2613 mmHg

FAQs on Vapour Pressure Formula

Question 1: What is Raoult’s law and its application?

Answer:

Raoult’s law states that the partial pressure of a pure component (liquid or solid) in a perfect mixture of liquids is equal to the vapour pressure of that component multiplied by the mole fraction in the mixture.

Raoult’s law is applicable:

  • To evaluate the reduction in non-volatile solute vapour pressure.
  • To evaluate the liquids’ ability to bind strongly.

Question 2: Write the Vapour Pressure formula.

Answer:

A solution is created when a solid is dissolved in a liquid. The resultant vapour pressure of this solution is decreased when the solute is introduced. Raoult’s law, which describes how the vapour pressure of a liquid changes when a solute is added, is used to calculate vapour pressure.

Psolution = (Xsolvent)×(P°solvent)

where,

Psolution is the vapour pressure of a solution 
Xsolvent is the mole fraction of solvent in a solution
P° solvent is the vapour pressure of a solvent

Question 3: For what type of solutions, Raoult’s law is valid?

Answer:

Raoult’s law is only valid for ideal solutions. The solvent-solute interaction is the same as a solvent-solvent or solute-solute interaction in an ideal solution. This suggests that both the solvent and the solute expend the same amount of energy to reach the vapour phase as they do in their pure states.

Question 4: Write the limitations of Raoult’s law.

Answer:

There are limitations of Raoult’s law that are mentioned below:

Since Raoult’s law applies to ideal solutions, i.e., those in which the gas phase exhibits thermodynamic features resembling a combination of ideal gases, it is particularly pertinent. The fact that they are uncommon and hard to find is the only negative. Because distinct attractive forces exist in many liquid mixes, certain solutions depart from Raoult’s equation.

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.

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CBSE Class 12 Chemistry Syllabus Download PDF

Below is the CBSE Class 12 Syllabus along with the marking scheme and time duration of the Chemistry exam.

S.NoTitleNo. of PeriodsMarks
1Solutions107
2Electrochemistry129
3Chemical Kinetics107
4d -and f -Block Elements127
5Coordination Compounds127
6Haloalkanes and Haloarenes106
7Alcohols, Phenols and Ethers106
8Aldehydes, Ketones and Carboxylic Acids108
9Amines106
10Biomolecules127
Total70

CBSE Class 12 Chemistry Practical Syllabus along with Marking Scheme

The following is a breakdown of the marks for practical, project work, class records, and viva. The total number of marks for all parts is 15. The marks for both terms are provided in the table below.

Evaluation Scheme for ExaminationMarks
Volumetric Analysis08
Salt Analysis08
Content-Based Experiment06
Project Work and Viva04
Class record and Viva04
Total30

CBSE Class 12 Chemistry Syllabus (Chapter-wise)

Unit -1: Solutions

  • Raoult's law.
  • Colligative properties - relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, osmotic pressure, determination of molecular masses using colligative properties, abnormal molecular mass.
  • Solutions, Types of solutions, expression of concentration of solutions of solids in liquids, solubility of gases in liquids, solid solutions.
  • Van't Hoff factor.

Unit -2: Electrochemistry

  • Redox reactions, EMF of a cell, standard electrode potential
  • Nernst equation and its application to chemical cells
  • Relation between Gibbs energy change and EMF of a cell
  • Kohlrausch's Law
  • Electrolysis and law of electrolysis (elementary idea)
  • Dry cell-electrolytic cells and Galvanic cells
  • Conductance in electrolytic solutions, specific and molar conductivity, variations of conductivity with concentration.
  • Lead accumulator
  • Fuel cells

Unit -3: Chemical Kinetics

  • Rate of a reaction (Average and instantaneous)
  • Rate law and specific rate constant
  • Integrated rate equations and half-life (only for zerfirst-order order reactions)
  • Concept of collision theory (elementary idea, no mathematical treatment)
  • Factors affecting rate of reaction: concentration, temperature, catalyst;
  • Order and molecularity of a reaction
  • Activation energy
  • Arrhenius equation

Unit -4: d and f Block Elements  

  • Lanthanoids- Electronic configuration, oxidation states, chemical reactivity and lanthanoid contraction and its consequences.
  • Actinoids- Electronic configuration, oxidation states and comparison with lanthanoids.
  • General introduction, electronic configuration, occurrence and characteristics of transition metals, general trends in properties of the first-row transition metals – metallic character, ionization enthalpy, oxidation states, ionic radii, color, catalytic property, magnetic properties, interstitial compounds, alloy formation, preparation and properties of K2Cr2O7 and KMnO4.

Unit -5: Coordination Compounds  

  • Coordination compounds - Introduction, ligands, coordination number, color, magnetic properties and shapes
  • The importance of coordination compounds (in qualitative analysis, extraction of metals and biological system).
  • IUPAC nomenclature of mononuclear coordination compounds.
  • Bonding
  • Werner's theory, VBT, and CFT; structure and stereoisomerism

Unit -6: Haloalkanes and Haloarenes  

  • Haloarenes: Nature of C–X bond, substitution reactions (Directive influence of halogen in monosubstituted compounds only). Uses and environmental effects of - dichloromethane, trichloro methane, tetrachloromethane, iodoform, freons, DDT.
  • Haloalkanes: Nomenclature, nature of C–X bond, physical and chemical properties, optical rotation mechanism of substitution reactions.

Unit -7: Alcohols, Phenols and Ethers   

  • Phenols: Nomenclature, methods of preparation, physical and chemical properties, acidic nature of phenol, electrophilic substitution reactions, uses of phenols.
  • Ethers: Nomenclature, methods of preparation, physical and chemical properties, uses.
  • Alcohols: Nomenclature, methods of preparation, physical and chemical properties (of primary alcohols only), identification of primary, secondary and tertiary alcohols, mechanism of dehydration, and uses with special reference to methanol and ethanol.

Unit -8: Aldehydes, Ketones and Carboxylic Acids   

  • Carboxylic Acids: Nomenclature, acidic nature, methods of preparation, physical and chemical properties; uses.
  • Aldehydes and Ketones: Nomenclature, nature of carbonyl group, methods of preparation, physical and chemical properties, mechanism of nucleophilic addition, the reactivity of alpha hydrogen in aldehydes, uses.

Unit -9: Amines    

  • Diazonium salts: Preparation, chemical reactions and importance in synthetic organic chemistry.
  • Amines: Nomenclature, classification, structure, methods of preparation, physical and chemical properties, uses, and identification of primary, secondary and tertiary amines.

Unit -10: Biomolecules     

  • Proteins -Elementary idea of - amino acids, peptide bond, polypeptides, proteins, structure of proteins - primary, secondary, tertiary structure and quaternary structures (qualitative idea only), denaturation of proteins; enzymes. Hormones - Elementary idea excluding structure.
  • Vitamins - Classification and functions.
  • Carbohydrates - Classification (aldoses and ketoses), monosaccharides (glucose and fructose), D-L configuration oligosaccharides (sucrose, lactose, maltose), polysaccharides (starch, cellulose, glycogen); Importance of carbohydrates.
  • Nucleic Acids: DNA and RNA.

The syllabus is divided into three parts: Part A, Part B, and Part C. Part A consist of Basic Concepts of Chemistry, which covers topics such as atomic structure, chemical bonding, states of matter, and thermochemistry. Part B consists of Topics in Physical Chemistry, which includes topics such as chemical kinetics, equilibrium, and electrochemistry. Part C consists of Topics in Organic Chemistry, which covers topics such as alkanes, alkenes, alkynes, and aromatic compounds.

Basic Concepts of Chemistry:

  • Atomic structure: This section covers the fundamental concepts of atomic structure, including the electronic configuration of atoms, the Bohr model of the atom, and the wave nature of matter.
  • Chemical bonding: This section covers the different types of chemical bonds, including ionic, covalent, and metallic bonds, as well as the concept of hybridization.
  • States of the matter: This section covers the three states of matter - solid, liquid, and gas - and the factors that influence their properties.
  • Thermochemistry: This section covers the principles of thermochemistry, including the laws of thermodynamics and the concept of enthalpy.

Chapters in Physical Chemistry:

  • Chemical kinetics: This section covers the study of the rate of chemical reactions and the factors that influence it, including the concentration of reactants, temperature, and the presence of catalysts.
  • Equilibrium: This section covers the principles of chemical equilibrium, including the concept of Le Chatelier's principle and the equilibrium constant.
  • Electrochemistry: This section covers the principles of electrochemistry, including the concept of half-cell reactions, galvanic cells, and electrolysis.

Chapters in Organic Chemistry:

  • Alkanes: This section covers the properties and reactions of alkanes, including their structure, isomerism, and combustion.
  • Alkenes: This section covers the properties and reactions of alkenes, including their structure, isomerism, and addition reactions.
  • Alkynes: This section covers the properties and reactions of alkynes, including their structure, isomerism, and addition reactions.
  • Aromatic compounds: This section covers the properties and reactions of aromatic compounds, including their structure, isomerism, and electrophilic substitution reactions.

In addition to the topics covered in the syllabus, the CBSE Class 12 Chemistry exam also tests students on their analytical and problem-solving skills, as well as their ability to apply the concepts learned in the classroom to real-world situations.

Students can also check out the Tips for the Class 12 Chemistry Exam. They can easily access the Class 12 study material in one place by visiting the CBSE Class 12 page at ANAND CLASSES (A School Of Competitions). Moreover, to get interactive lessons and study videos, download the ANAND CLASSES (A School Of Competitions) App.

Frequently Asked Questions on CBSE Class 12 Chemistry Syllabus

Q1

How many chapters are there in the CBSE Class 12 Chemistry as per the syllabus?

There are 10 chapters in the CBSE Class 12 Chemistry as per Syllabus. Students can learn all these chapters efficiently using the study materials provided at ANAND CLASSES (A School Of Competitions).

Q2

What is the marking scheme for CBSE Class 12 Chemistry practical exam according to the syllabus?

The marking scheme for CBSE Class 12 Chemistry practical exam, according to the syllabus, is 8 marks for volumetric analysis, 8 marks for salt analysis, 6 marks for the content-based experiment, 4 marks for the project and viva and 4 marks for class record and viva.

Q3

Which is the scoring chapter in Chemistry as per CBSE Class 12 syllabus?

The chapter Electrochemistry in Chemistry is the scoring chapter as per CBSE Class 12 syllabus.

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