Solutions

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Solutions

SolutionsTypes of Mixtures:Heterogeneous mixtures: not a definite or uniform compositionExample: soil, milkHomogeneous mixtures:Uniform appearanceExample: SaltwaterSalt and chlorine are interspersed among the water molecules. (Dissolved)Salt- pure substanceWater- pure substanceboth have definite compositionSaltwater- mixture- uniform, but not a definite composition.Salt dissolves in water. Salt is described as “soluble in water.” By soluble we meancapable of being dissolved.(becomes evenly distributed among the water molecules)Even salty tasteDissolving of Salt in waterA solution is a homogeneous mixture of two or more substances ina single phase.Characteristics of Solutions:Do not separate on standingCannot be separated by filtrationDo not scatter lightComponents of SolutionsDissolving medium in a solution: solventSubstance dissolved in a solution: solute.There can be one or more solutes.The solvent is always the one with the most volume.Solutes: Electrolytes Versus Nonelectrolytes•A substance that dissolves in water to give a solution that conducts electric current iscalled an electrolyte.•Any soluble ionic compound, such as sodium chloride, NaCl, is an electrolyte.•The positive and negative ions separate from each other in solution and are free tomove, making it possible for an electric current to pass through the solution.


A substance that dissolves in water to give a solution that does not conduct electriccurrent is called a nonelectrolyte.Sugar = nonelectrolyte.Neutral solute molecules do not contain mobile charged particles, so a solution of anonelectrolyte cannot conduct electric current.Factors Affecting the Rate of DissolutionSurface area of the solute is increased.Stirring or shakingHigher temperaturesSolubilityIf you add spoonful after spoonful of sugar to tea, eventually no more sugar will dissolve.There is a limit to the amount of solid that can be dissolved.The point at which this limit is reached for any solute-solvent combination depends on thenature of the solute, the nature of the solvent, and the temperature.SolubilityWhen a solute is first added to a solvent, solute molecules leave the solid surface andmove about at random in the solvent.As more solute is added, more collisions occur between dissolved solute particles. Someof the solute molecules return to the crystal.


When maximum solubility is reached, molecules are returning to the solid form at thesame rate at which they are going into solution.Solution equilibrium is the physical state in which the opposing processes of dissolutionand crystallization of a solute occur at the same rates.Unsaturated solutions contain less than the maximum amount of solute possible. Asmore solid is added to this solution, it will dissolve.Saturated solutions contain all the solute that is possible to dissolve under a given set ofconditions. As more solid is added to this solution, it will not dissolve.Supersaturated solutions are unstable solutions that have been forced to dissolve moresolute than should be possible under a given set of condition. A crystal placed into asupersaturated solution will begin to grow.The solubility of a substance is the amount of that substance required to form asaturated solution with a specific amount of solvent at a specified temperature.example: The solubility of sugar is 204 g per 100 g of water at 20°C.Solubilities vary widely, and must be determined experimentally.They can be found in chemical handbooks and are usually given as grams of solute per 100 g ofsolvent at a given temperature.S


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Nonpolar solventsIonic compounds are generally not soluble in nonpolar solvents such as carbontetrachloride, CCl 4 , and toluene, C 6 H 5 CH 3 .Two polar substances, or two nonpolar substances, on the other hand, form solutionstogether easily because their intermolecular forces match.“Like dissolves like”Miscible: two liquids that dissolve in each other.Immiscible: two liquids that are not soluble in each other.Tincture: a solution with an alcohol solventEffects of Pressure on SolubilityChanges in pressure have no effect on the solubilities of liquids or solids in liquidsolvents.However, increases in pressure increase gas solubilities in liquids.Increasing the temperature usually decreases gas solubility.Increasing the temperature usually increases solubility of solids in liquids.Concentration: The amount of solute dissolved in a solvent•The concentration of a solution is a measure of the amount of solute in a given amountof solvent or solution.•Concentration is a ratio.Molarity•Molarity is the number of moles of solute in one liter of solution.•For example, a “one molar” solution of sodium hydroxide contains one mole of NaOH inevery liter of solution.•The symbol for molarity is M. (read “molar”) The concentration of a one molar NaOHsolution is written 1 M NaOH.To calculate molarity, you must know the amount of solute in moles and the volume ofsolution in liters.example: One mole of NaOH has a mass of 40.0 g. If this quantity of NaOH isdissolved in enough water to make 1.00 L of solution, it is a 1.00 M solution.The molarity of any solution can be calculated by dividing the number of moles of soluteby the number of liters of solution:M =mol soluteL solutionSample Problem:You have 3.50 L of solution that contains 90.0 g of sodium chloride, NaCl. What is the molarity ofthat solution?


M =mol soluteL solutionM = 90.0 g3.50 L ! 1 mol58.44 gM = 0.440 M NaClConcentration: Percent by massPercent by mass is the ratio of mass units of solute to mass unitsof solution, expressed as a percent.! massof solute $percent by mass= #&x100" massof solution %Concentration: Percent by volume! volumeof solute $percent by volume= #&x100" volumeof solution %Concentration: Parts per million (ppm)! massof solute $ppm= #&x10 6" massof solution %Concentration: Parts per billion (ppb)! massof solute $ppb= #&x10 9" massof solution %Molality (m)Molality = mol solutekilogram solventThe symbol for molality is a lower case munits = mol/kgSample Problem: Vanillin (C 8 H 8 O 3 ) is a flavoring agent. A 37.2 g sample of vanillin wasdissolved in 168.5 g of diphenyl ether. What is the molality of the solution?mol solutem =kg solventhint: the solute will always be the smallest amount.m =! 37.2 g $ !# & 1000 g $ ! 1 mol $# &#&" 168.5 g %"1 kg %"152.16 g %m = 1.45 m(mol/kg)


DilutionsMany times in the laboratory it is necessary to use a concentratedsolution and dilute it to a different molarity.M 1 V 1 = M 2 V 2M – MolarityV – VolumeSample Problem: You need a 4.0 L solution of 0.10 M sodiumhydroxide. How many milliliters of concentrated 6.0 M solution willyou need to make the dilution?V1 = M2V2M1( 0.10 M) 4.0 LV1 =6.0 MV1 = 0.067 L( )

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