Properties of Solutions 11.5-11.8

Boiling-Point Elevation and Freezing-Point Depression

Colligative Properties Properties that are ONLY dependent on the number of solute particles Not dependent on their identity Examples Boiling-point elevation Freezing-point depression Osmotic pressure

Boiling-Point Elevation Nonvolatile solutes elevate the boiling-point of the solvent ΔT= Kbmsolute ΔT - boiling point elevation Kb - the molal boiling point elevation constant of the solvent msolute – the molality of the solute in the solutions

Freezing-Point Depression Solutes depress the freezing point of the solvent ΔT= Kfmsolute ΔT - freezing point depression Kf - the molal freezing point depression constant of the solvent msolute – the molality of the solute in the solutions

Osmotic Pressure Osmosis – flow of solvent molecules into a solution through a semi-permeable membrane Small concentration of solute produces a relatively large osmotic pressure

Osmotic Pressure The pressure necessary to keep water from flowing across a semi-permeable Osmotic pressure can be used to characterize solutions and determine molar masses π = MRT π – osmotic pressure in atmospheres M – molarity of the solution R – gas law constant T – Kelvin temperature

Osmosis Dialysis – transfer of solvent molecules as well as small solute molecules and ions Isotonic Solutions – solutions that have the same osmotic pressure Osmotic Pressure in Living Cells Crenation – cells placed in a hypertonic solution lose water to the solution and shrink Hemolysis – cells placed in a hypotonic solution gain water from the solution and swell, possibly bursting

Reverse Osmosis External pressure applied to a solution can cause water to leave the solution Concentrates impurities (salt) in the remaining solution Pure solvent (water) is recovered on the other side of the semi-permeable layer

Colligative Properties of Electrolyte (Ionic) Solutions

van’t Hoff Factor (i) van’t Hoff Factor (i) – the relationship between the moles of solute dissolved and the moles of particles in solution i = moles of particle in soln moles of solute dissolved For ionic compounds, the expected value of i is an integer greater than 1 NaCl – 2, BaCl2 – 3, etc.

van’t Hoff Factor (i) Values of i are less than expected due to ion pairing Ion pairing – phenomenon occurring in solution when oppositely charged ions aggregate and behave as a single particle Closer to expected values in more dilute solutions

van’t Hoff Factor (i) Incorporating the van’t Hoff Boiling point/freezing point ΔT = imK Osmotic Pressure π = iMRT

Colloids Colloid – suspension of particles in some medium Particles do no precipitate out because of electrostatic repulsion Tyndall effect – the scattering of light by particles and is used to distinguish between a suspension and a true solution Light passes through a solution Light is scattered in a colloid