Solution Notes

Page # 25 SOLUTION SOLUTION Solution : Homogeneous mixture of two or more substances. Concentration of solution: Let solute = A, solvent = B WA Mass percentage (w/w): of A = W  W × 100 A B VA Volume percentage (V/V) : of A = V  V ×100 A B Molarity (M): No of moles of solute per litre of solution WA  1000 A M = V = M  Vs A (ml) s Molality(m)  No. of moles of solute present in 1 kg of solution. A W A  1000 m= W = M W B(kg) A B( g ) Mole fraction: The ratio of no of moles of solute or solvent to the total no of moles of the solution . A XA =    A B , B XB =    A B XA + XB = 1 Parts per million (ppm): The number of parts by weight of solute in 1 million parts by weight of the solution. WA 6 ppmA = W  10 S Solubility of gases in liquids -depends on (a) Nature of gas (b) Nature of liquid (c) Temperature of solution  solubility  1 Temperature (d) Pressure  solubility  Pressure Henry's law - The solubility of a gas in a liquid at a given temperature is directly proportional to the partial pressure of the gas over the solution. m  p  m = K HP According to Dalton - Expressing solubility in terms of mole fraction. for gas A P A  xA  PA = KHXA (PA = partial Pressure) Solubility of solids in liquids - depends on . (a) Nature of solute and solvent (b) temperature - Solubility  temperature ( For endothermic) Solubility  1 Temperature ( For exothermic) Vapour pressure - The pressure exerted by the vapours of a liquid in equilibrium with the liquid at a particular temperature. 1 Vapour pressure  Temperature  Inter molecular forces among liquid moleculers But if we add non volatile solute in the solution, the vapour pressure decreases. Page # 26 SOLUTION Raoult law : The vapour pressure of a solution containing a volatile solute is directly proportional to mole fraction of the solvent. PA  xA. OR "The relative lowering of vapour pressure of solution having non volatile solute is equal to the mole fraction of solute" PAo  PA PAo = XB Ideal solution: Those solutions which obey Raults low over entire range of concentration. In ideal solution intermolecular interaction between the two component (A B) are of same magnitude as the intermolecular interaction between pure components A A and B B. Charsctersties of ideal solutions (1) obey Raoult law (2) mix H = 0 (3) Vmix = 0 Non ideal solution: The solution which do not obey Raoults low over entire range of concentrations are called non ideal solution. In these solutions intermolecular interaction between the 2 components (A B) are of different magnitude as the intermolecular interactions between pure components A A and B B. Characterstics of non ideal solution: 1. Does not obey Raoults law 2. Hmix  0 3. Vmix  0 Two Types xºB xºB Vapour pressure Negative deviations Vapour pressure Positive deviations xºA xA=0 Mole fraction xB=1 xA Interaction A BA xA=1 xB =0 A and B B Characterstics PA > PºA XA and PB > PºB XB. PA < PºA XA and PB < PºB XB. Hmix < 0 Hmix < 0 Vmix > 0 Vmix < 0 Examples benzene and acetone Acetone and Chloroform or aniline Acetone and ethyl alcohol Acetic acid and Pyridine Colligative properties Those properties of a solution which depend on the number of solute particles only but not on the nature of the solute are called colligative properties. These are (a) Relative lowering of vapour pressure of a solution is equal to the mole fraction of the solute . PAo  PA PAo WA / M A = XB = W / M B B Page # 27 SOLUTION (b) Elevation in boiling point Tb  m  Tb  Kbm  (c) Depression in feezing point Tf m Tb  Kfm (d) Osmotic pressure: The pressure which must be applied to the solution side to just prevent the passage of pure solvent into it through a semipermeable membrane  = CRT.  W RT   A RT  A V MA V Azeotropes:Those solutions which have same composition in liquid and vapour phase Two, Types Minimum boiling Azeotropes : Non -ideal solution showing laye positive deviation from Raoults law form min boiling azeotropes which boil at temperature lower than boiling point of its components eg. water and benzene, chloroform and methanol. Maximum boiling Azeotropes : Non -ideal solutions showing negative deviation from Raoults law They boil at temperature higher than the boiling point of its components eq. mixture of HCl and H2O containing 20.2% of HCl by weight boils a t 108.5ºC higher than either pure HCl (85ºC) or water (100ºC). eq. 68% HNO3 and 32% water by mass (b.p. 393.5 k ) Reverse Osmoses : If a pressure more than osmotic pressure is applied on the solution side, the solvent will flow from the solution side to solvent side a through semipermeable membranes. This phenomenon is called Reverse osmosis Example – Disalination of sea water. Abnormal molecular mass: Sometimes the observed molecular mass of a substance determined with the help of colligative properties is quite different as compared to the normal molecular mass. This is called abnormal molecular mass. It is mainly due to the dissociation or association of the solute in the solution. Van't Hoff factor(i) : It is the ratio of normal molecular mass to the abnormal molecular mass. Or The ratio of observed colligative property to the normal or calculated colligative property i= Normal molecular mass , Abnormal molecular mass i= Observed colligative property Normal Coligative property i = 1 (neither association or dissociation) i > (for dissociation) i < (for association) Degree of Association  = mi  1 [m = no. of moles associated] lm Degree of Dissociation  = i  1 [m = no. of moles dissociated] m 1 Modified Colligative properties: The inclusion of van't Hoff factor modifies the equation for the coagulative properties as follows : PAo  PA PAo = i. XB. Tf = iKfm Tb = iKbm  = iCRT.