Properties of Solutions

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Properties of Solutions

11.5-11.8

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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

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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