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Solutions

In our daily life we come across the solutions like tonics, salt solutions, alloys (Solid solutions) like brass, bronze, german silver so on. A solution is a homogeneous mixture of 2 or more chemically non reacting substances. A binary solution is a homogeneous mixture of solvent (major part) and solute (minor part). Concentration of a solution is expressed by different methods. Some of those methods are:

Molarity (M): Number of gram moles of solute present in 1 litre solution.
                       
                     Units: gram Moles/lit.
Normality (N): Number of gram equivalents of solute present in 1 litre solution.
                      
                        Units: gram equivalents/lit.
Molality (m): Number of gram moles of solute present in 1 kg solvent.
                   
                        Units: gram Moles/kg
* One molar (1 M) aqueous solution is more concentrated than one molal (1 m) aqueous solution.


Mole fraction (X): It is the ratio between the number of moles of one component (solute or solvent) to the total number of moles of all the components of the solution. It has no units.
                   
          M, N involve volumes, which change with temperature. m, X involve masses, which does not change with temperature.


Mass percentage 


The mass of the component present in 100 grams of the solution.


Volume percentage 
The volume of the component present in 100 ml of the solution.


 Mass by Volume percentage 
The mass of the component present in 100 ml of the solution.


Parts per million (ppm):


Number of parts of one component present in one million parts of the solution.


SOLUBILITY 
       The maximum amount of a substance that can be dissolved in a specified amount of solvent at given temperature is called solubility. It depends upon pressure, temperature, nature of solute & solvent.
        Mole fraction of gas in the solution  partial pressure of the gas (Dalton's law).
        According to Henry's law "At a given temperature, the partial pressure of a gas (p) [in vapour phase] is directly proportional to the mole fraction of the gas (x) in the solution".
       p  x
   p = KH . x Where KH = Henry's law constant.


Applications of Henry's law:
* Dissolution of CO2 under high pressure in soft drinks and soda water to give good taste.
* Usage of mixture of 11.7% Helium, 56.2% N2 and 31.1% Oxygen by Scuba divers to get rid of the worst condition "bends".
* "Anoxia", becoming weak and unable to think clearly by the climbers and people living at high altitudes due to low concentration of O2 in the blood.
Like in gases, molecules of liquid also move freely with different velocities. Due to exchange of Kinetic energy during collisions, the molecules at the surface having high Kinetic energy escape into space. This process is called ''Vaporisation''.
        The reverse process is called "Condensation". At a given temperature, the pressure exerted by the vapour over the surface of liquid when the vapour is in equilibrium with the liquid is known as "Vapour Pressure" (V.P). V.P. depends on nature, surface area of the liquid, flow of air over the surface and temperature. The temperature at which V.P. becomes equal to the atmospheric pressure is known as "Boiling Point" (B.P.).
       When a non volatile solute is dissolved in a pure solvent, V.P. of pure solvent (Po) decreases. This decrease is known as "Lowering of Vapour Pressure" (L.V.P.), equal to Po − Ps. The ratio of L.V.P. and Po is known as "relative lowering of vapour pressure" (R.L.V.P.), equal to.  According to Raoult's law, "The R.L.V.P. of a solution is equal to the mole fraction of non volatile solute (X2)".
                     =  X2.
R.L.V.P. is independent of temperature. A solution which obey Raoult's law at all concentrations and temperatures is known as "ideal solution". This law is applicable to dilute solutions, ideal solutions, solutions having non volatile solute, solute does not under go either association or dissociation.
      The enthalpy change of mixing of the pure components to form the solution is zero (∆mix H = 0) and the volume change of mixing is also zero (∆mix V = 0).
      When a solution does not obey Raoult's law at all the concentrations, it is called non-ideal solution. If the vapour pressure of this type of solution is higher than that of predicted by Raoult's law. It exhibits positive deviation and if lower, it is called negative deviation from Raoult's law.
     

 In a solution showing the positive deviation, the attractive forces between molecules of A and B is less than between A −A & B − B molecules. For this type of solution ∆ Hmix > 0 and ∆ Vmix > 0, p > pA + pB.
          In a solution showing the negative deviation, the attractive forces between molecules of A and B is more than between A −A & B − B molecules. For this type of solution ∆ Hmix > 0 and ∆Vmix > 0, p > pA + pB.
         Some liquids on mixing, form AZEOTROPES", in which the composition of liquids remain same in liquid phase and as well as in vapour phase and boil at constant temperature. The solution which show large positive deviation from Raoult's law form "minimum boiling Azetrope" (e.g.: Ethanol - Water mixture). The solution which show large negative deviation from Raoult's law form "maximum boiling azetrope" (e.g.: Nitric acid - Water mixture).
         According to Dalton's law of partial pressures, total pressure (ptotal) over the solution phase is equal to the sum of the partial pressures of the component gases.    
                 ptotal = p1 + p2
                            = x1 p10 + x2 p20
Some times properties of dilute solution depends on the number of solute particles but not on the nature of solute are called "Colligative properties".
They are ....


a) Relative lowering of vapour pressure
b) Elevation of boiling point (ΔTb): The difference between b.p of solution (Tb) and b.p. of pure solvent (Tbo). 
       Δ Tb = Tb − Tbo                        Δ Tb = Kb.m
     

      Kb (molal elevation constant): The elevation in b.p. observed in one molal solution.


c) Depression of freezing point (ΔTf): The difference between F.P. of pure solvent (Tfo) and F.P. of solution (Tf ).
       Δ Tf  = Tf o − Tf                                  Δ Tf  = Kf .m
                                             
       Where w1, w2 are weight of solvent and weight of  solute  respectively. M2 is molar mass of the solute.
      Kb = molal elevation constant (Ebullioscopic constant)
      Kf = molal depression constant (Cryoscopic constant)


Kf (molal depression constant): The depression in freezing point observed in one molal solution. This is independent of chemical nature of solute, but depends on the chemical nature of solvent.


d) Osmotic pressure (π): The pressure required to be applied on the solution to prevent the osmotic flow of solvent into solution through a semi permeable membrane.
     π = C.R.T.
      The solutions having same osmotic pressure under similar conditions are known as "Isotonic Solutions". 0.9%   NaCl (Saline) solution is isotonic with blood. If the solution having lower osmotic pressure (< 0.9%

NaCl), it is known as "Hypotonic Solution". If the solution having higher osmotic pressure ( > 0.9%  NaCl), it is known as "Hypertonic Solution". If π of the contents of the living cell is not equal to that of contents surrounding it outside, "Haemolysis" (Contents from outside enters into cell, the cell bulges and finally bursts) and "Plasmolysis" (Contents from the cell may come out of the cell, the cell shrinks and collapses) takes place.
      One can determine molar mass of solute by using colligative properties R.L.V.P. by Ostwald's dynamic method, Δ Tb by Cottrell's method, Δ Tf by Rast's method and π by Berkeley Hartley method. In Rast's method both the solvent (Camphor, having high Kf) and solute are solids.

olligative properties are shown by ideal solutions. As the concentration of solution increases, deviations from Raoult's law are observed. These deviations are due to dissociation (increase in colligative property & number of particles, i > 1) or due to association (decrease in colligative property & number of particles, i < 1). As the colligative property's inversely proportional to molar mass of the solute, molar mass determined by colligative property may be less or more than the theoretical molar mass. Hence Van't Hoff introduced a factor "i" in the equation to equalize the theoretical molar mass and experimental molar mass from the equation of colligative property. Hence they must be calculated with "i".
          Δ Tb = i. Kb.m,                      Δ Tf = i.Kf.m,                        π = i.CRT
   For ideal solutions i = 1
     i = Van't Hoff factor
      

   For solute dissociation process,   dissociation   =   
                               

Posted Date : 06-08-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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