Solutions

Raoult's Law for Binary Solution of Two Liquids: 1. For a solution of two miscible liquids, the total vapour pressure of the solution is equal to the sum of the partial vapour pressure of the two liquids. P = PA + PB PA = and PB = Composition of vapour: Mole fraction of A in vapour phase YA = and YB = Note: When vapours over a solution are condensed, the mole fraction of a component in vapour phase is equal to mole fraction of that component in condensate. Ideal Solutions: The solution which obey Raoult's law at all compositions of solute in solvent at all temperature are called ideal solutions. For ideal solution: 1. PA = XAPAo , PB = XBPBo 2. Hmix = 0 3. Vmix = 0 Colligative Properties: The properties of dilute solutions which depend only on number of particles of solute are called colligative properties. Colligative properties are of four types 1. Lowering of vapour pressure 2. Elevation of Boiling point 3. Depression in Freezing point 4. Osmotic Pressure Lowering of Vapour Pressure Raoult's Law For Realtive Lowering of Vapoure Pressure Relative lowering of vapour pressure of a solution containing non volatile solute is equal to the mole fraction of solute present in the solution. Where, = V.P. of solvent P1 = V.P. of solution n = number of moles of solute N = number of moles of solvent Elevation of Boiling Point: Tb m or Tb = Kb m Tb - elevation in boiling point Kb - molal boiling point elevation constant or Ebullioscopic constant. m - molality of the solution Molal boiling point elevation constant or ebullioscopic constant of the solvent, is defined as the elevation in boiling point which may theoretically be produced by dissolving one mole of any solute in 1000 g of the solvent. or , where m1 = molecular weight of solute and w and W are weights of solute and solvent. Note: KB is constant for a given solvent. Value of Kb can be given by Kb = RTb2/1000 lv where R is gas constant in ca, Tb is boiling point of solvent in K and lv is latent heat of vapourisation of solvent in cal per gm. Depression in Freezing Point: Tf m or Tf = Kf m Tf - depression in freezing point Kf - molal depression constant or cryoscopic constant. m - molality of the solution Molal freezing point depression constant of the solvent or cryoscopic constant, is defined as the depression in freezing point which may theoretically be produced by dissolving 1 mole of any solute in 1000g of the solvent. or where m1 = molecular weight of solute and w and W are weights of solute and solvent. Note: Similar to Kbthe value of Kf depends on solvent nature. Kf= RTf2/1000 lfwhere R is gas constant in cal. Tf is freezing point of pure solvent and lf is the latent heat of fusion in cal/g. Osmotic Pressure: V = nRT - osmotic pressure V - volume of solution T - temperature R - gas constant n - no. of moles of solute Isotonic Solution: Two solutions having equal osmotic pressure are called isotonic solution n1 / V1 = n2 / V2 Abnormal Molecular Weight and Van't Hoff Factor: Since colligative properties depend upon the number of particles of the solute, in some cases where the solute associates or dissociates in the solution, abnormal results for molecular mass are obtained. Van't Hoff factor i) = = i) For association i = 1- (1-1/2) ( i < 1) = degree of association n = number of molecules associated ii) For dissocation i = 1+(n-1) (i > 1) = degree of dissociation n = number of particles formed iii) When molecular form of solute does not change, i =1