Vapur Pressure Explained In Simple Terms

What is the Vapour Pressure?

Vapur pressure is the pressure exerted by vapours of solution at surface of solution at equilibrium.

There are two scenarios where Vapur Pressure work:

First, It should be Liquid-Liquid Solution.

In this type of solution, solid is solute and a liquid is solvent. Concentration of solid can be counted in three ways: Mass %, Parts per million (PPM), and Parts per billion (PPB). Apart from that, this solution is prepared by dissolving an fixed concentration of solid in a solvent (if a solvent isn't mentioned, consider it water). Also, solvent are of two types: Polar solvent and Non-Polar Solvent. 

Polar solvent is the type of solvent in which dipole moment is non-zero whereas Non-polar solvent has zero dipole moment. Generally, non-polar solvent have central atoms with no lone pair, making it without dipole moment. Polar moment are further categorized in two ways: Protic and Aprotic Polar solvent. In protic solvent: there are dissociable hydrogen atom whereas Non-aprotic solvent doesn't have. 

Furthermore, when a solution is prepared, it is left as it is. After some time, vapours are formed (due to evaporation) and these vapour exerts some pressure. 

Formula:               Pa = P'a x Xa

                                where VP/Pa = vapour pressure of one solution

                                            P'a = Standard Vapour pressure of solution

                                            Xa= Mole fraction of solvent.

Some points: P'a is directly proportional to the temperature and is inversely proportional to the force of attraction. Force of attraction in Polar sovent > those of non polar solvent, so V.P is higher in non polar solvent. Also, if we compare two non- polar solvent, solvent will lower molcular weight has higher Vapour pressure.  

Also, when we compare VP of different solutions, in that regard, lowest boiling point solution will have highest vapour pressure.

If there are two solution: 

Formula:             Pt= VPa + VPb => Pt = P'a x Xa + P'b x Xb

Above formula can be written as : Pt = (P'a - P'b)Xa + P'b or Pt = (P'b - P'a)Xb + P'a

Also, total vapour pressure will be between P'a and P'b. 

                                            Highest Pt = P'a

                                            Lowest Pt = P'b

Also, 1 atm = 760 mm hg = 760 torr. I atm pressure means that liquid is on boiling point.

Mole fraction of Vapours

Ya = Pa/Pt => P'aXa/(P'aXa + P'bXb)

llrly for Yb

Ya/Yb = (P'a/P'b)/Xa/Xb

                    If P'a > P'b                                              |    If P'a < P'b    

                    Ya/Yb > Xa/Xb                                       |     Ya/Yb < Xa/Xb

Second is solid-liquid solution.

If a solid solute/ non volatile solute is added, vapour pressure decreases. This is called Lowering of Vapour Pressure (P' - Ps = Xa) and then relative lowering of vapour pressure ((P' - Ps)/P' = Xa) 

Other useful formula: (Ps - P')/Ps = Na/Nb (where Na is no o moles of solute and Nb is the no. of moles of solvent)

Due to lowering of vapour pressure, two effects can be observed: Elevation in boiling point and Depression in freezing point. 

Elevation of boiling pt.

Boiling point is increased when solid solute is added

                    dTb = Kb x m (where dT is the change in temerature and Kb is ebullioscopic constant)

                        Kb = (R x Tb^2)/ 1000 x dL

                             or (R x Tb^2 x Molecular weight of solvent)/ 1000 x dH

                dL  = Latent heat of vapourisation: total amount of energy required to change I gram of solid                             to gas

                dH =  Enthalpy of vapourization: Total amount of energy required to change 1 mole of liquid                             to gas.

Depression of freezing point.

When a non volatile solute i.e. solid solute is added, freezing point of solution decreases.

                                                dTf = Kf x molality

                                                Kf = (R x Tf^2 x Molecular weight of solvent)/ 1000 x dHfusion

                                dH = Enthalp of fusion: Amount of energy released in freezing of 1 mole of liquid