Concentrations of Solutions and Colligative Properties

Mass Percentage mass of A in solution Mass % of A = total mass of solution Mass % of A =  100

Parts per Million and Parts per Billion Parts per Million (ppm) mass of A in solution total mass of solution ppm =  106 Parts per Billion (ppb) mass of A in solution total mass of solution ppb =  109

Molality and Mole Fraction 16.4 Molality and Mole Fraction The unit molality (m) is the number of moles of solute dissolved in 1 kilogram (1000 g) of solvent. Molality is also known as molal concentration.

Molality (m) mol of solute m = kg of solvent Because neither moles nor mass change with temperature, molality (unlike molarity) is not temperature dependent.

Molarity (M) mol of solute M = L of solution Because volume is temperature dependent, molarity can change with temperature.

total moles in solution Mole Fraction (X) moles of A total moles in solution XA = In some applications, one needs the mole fraction of solvent, not solute—make sure you find the quantity you need!

Colligative Properties Colligative properties depend only on the number of solute particles present, not on the identity of the solute particles. Among colligative properties are Vapor pressure lowering Boiling point elevation Melting point depression Osmotic pressure

Vapor Pressure As solute molecules are added to a solution, the solvent become less volatile (=decreased vapor pressure). Solute-solvent interactions contribute to this effect.

Vapor Pressure Therefore, the vapor pressure of a solution is lower than that of the pure solvent.

Vapor-Pressure Lowering 16.3 Vapor-Pressure Lowering Three moles of glucose dissolved in water produce 3 mol of particles because glucose does not dissociate. Particle concentrations differ for dissolved covalent and ionic compounds in water. a) Three moles of glucose dissolved in water produce 3 mol of particles because glucose does not dissociate. b) Three moles of sodium chloride dissolved in water produce 6 mol of particles because each formula unit of NaCl dissociates into two ions. c) Three moles of calcium chloride dissolved in water produce 9 mol of particles because each formula unit of CaCl2 dissociates into three ions.

Vapor-Pressure Lowering 16.3 Vapor-Pressure Lowering Three moles of sodium chloride dissolved in water produce 6 mol of particles because each formula unit of NaCl dissociates into two ions. Particle concentrations differ for dissolved covalent and ionic compounds in water. a) Three moles of glucose dissolved in water produce 3 mol of particles because glucose does not dissociate. b) Three moles of sodium chloride dissolved in water produce 6 mol of particles because each formula unit of NaCl dissociates into two ions. c) Three moles of calcium chloride dissolved in water produce 9 mol of particles because each formula unit of CaCl2 dissociates into three ions.

Vapor-Pressure Lowering 16.3 Vapor-Pressure Lowering Three moles of calcium chloride dissolved in water produce 9 mol of particles because each formula unit of CaCl2 dissociates into three ions. Particle concentrations differ for dissolved covalent and ionic compounds in water. a) Three moles of glucose dissolved in water produce 3 mol of particles because glucose does not dissociate. b) Three moles of sodium chloride dissolved in water produce 6 mol of particles because each formula unit of NaCl dissociates into two ions. c) Three moles of calcium chloride dissolved in water produce 9 mol of particles because each formula unit of CaCl2 dissociates into three ions.

Boiling Point Elevation and Freezing Point Depression Solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.

Boiling-Point Elevation 16.3 Boiling-Point Elevation The magnitude of the boiling-point elevation is proportional to the number of solute particles dissolved in the solvent. The boiling point of water increases by 0.512°C for every mole of particles that the solute forms when dissolved in 1000 g of water.

Boiling Point Elevation The change in boiling point is proportional to the molality of the solution: Tb = Kb  m where Kb is the molal boiling point elevation constant, a property of the solvent. Tb is added to the normal boiling point of the solvent.

Freezing Point Depression The change in freezing point can be found similarly: Tf = Kf  m Here Kf is the molal freezing point depression constant of the solvent. Tf is subtracted from the normal freezing point of the solvent.

Boiling Point Elevation and Freezing Point Depression In both equations, T does not depend on what the solute is, but only on how many particles are dissolved. Tb = Kb  m Tf = Kf  m

Colloidal Suspensions: Suspensions of particles larger than individual ions or molecules, but too small to be settled out by gravity.

Colligative Properties of Electrolytes However, a 1 M solution of NaCl does not show twice the change in freezing point that a 1 M solution of methanol does. It doesn’t act like there are really 2 particles.

van’t Hoff Factor One mole of NaCl in water does not really give rise to two moles of ions.

van’t Hoff Factor Some Na+ and Cl− reassociate as hydrated ion pairs, so the true concentration of particles is somewhat less than two times the concentration of NaCl.

The van’t Hoff Factor Tf = Kf  m  i We modify the previous equations by multiplying by the van’t Hoff factor, i Tf = Kf  m  i i = 1 for non-elecrtolytes

Osmosis Semipermeable membranes allow some particles to pass through while blocking others. In biological systems, most semipermeable membranes (such as cell walls) allow water to pass through, but block solutes.

Osmosis In osmosis, there is net movement of solvent from the area of higher solvent concentration (lower solute concentration) to the are of lower solvent concentration (higher solute concentration). Water tries to equalize the concentration on both sides until pressure is too high.

Osmotic Pressure n  = ( )RT = MRT V The pressure required to stop osmosis, known as osmotic pressure, , is n V  = ( )RT = MRT where M is the molarity of the solution If the osmotic pressure is the same on both sides of a membrane (i.e., the concentrations are the same), the solutions are isotonic.

Osmosis in Blood Cells If the solute concentration outside the cell is greater than that inside the cell, the solution is hypertonic. Water will flow out of the cell, and crenation results.

Osmosis in Cells If the solute concentration outside the cell is less than that inside the cell, the solution is hypotonic. Water will flow into the cell, and hemolysis results.

Suspensions Suspensions are mixtures where the particles do not stay suspended indefinitely Diameter is <1000nm (solution particles are about 1 nm) Particles settle out http://rlv.zcache.com/if_you_are_not_part_solution_suspension_humor_postcard-p239457398973765227envli_400.jpg (The Supernate is the water above the particles of a precipitate)

The Tyndall Effect Colloidal suspensions can scatter rays of light. This phenomenon is known as the Tyndall effect.

Colloids Colloids have particles bigger than in a solution but smaller than in a suspension. Particles won’t settle out over time Examples: whipped cream, marshmallow, milk, mayo, fog, aerosols, jellies, paint, blood, gelatin http://www.thelifevicarious.com/.a/6a00d83451c19f69e2015435bbf143970c-800wi

youtube video clip Brownian Motion Brownian Motion is the vibration of particles in a colloidal suspension youtube video clip Brownian Motion http://www.youtube.com/watch?v=6VdMp46ZIL8

the science of mayonnaise- an emulsion

Colloids in Biological Systems Some molecules have a polar, hydrophilic (water-loving) end and a nonpolar, hydrophobic (water-hating) end.

Colloids in Biological Systems Sodium stearate is one example of such a molecule.

Colloids in Biological Systems These molecules can aid in the emulsification of fats and oils in aqueous solutions.

Sometimes Tyndall effect Solution .1-1 nm Not able to filter No Tyndall Effect Colloid 1 – 1000nm Sometimes Tyndall effect Suspension < 1000 nm Will filter