Content Outline: Colligative Properties Osmotic Pressure (6.10)

Content Outline: Colligative Properties Osmotic Pressure (6.10)

Unit 6: Solution Chemistry

Content Outline: Colligative Properties – Osmotic Pressure (6.10)

  1. Diffusion
  1. The passive (no energy required) movement of molecules from an area of high concentration to an area of low concentration until equilibrium (same rates of movement) is establish within the entire area.

You should remember this from your Biology class when you were discussing cells moving molecules (such as food, waste, gases) across the phospholipid bi-layer (cell membrane).

1. [High]  [low] Remember, the brackets represent concentration.

  1. Osmosis (This is the movement of water across a membrane)
  1. This is the passive (no energy required) movement of solvent (Water for solutions) molecules through a semi-permeable membrane, such as found surrounding living cells.
  1. Semipermeable membrane -

a. This is a structure that allows the free passage of some particles while preventing other molecules from passing.

  1. Water has no concentration. So we use the term osmotic potential (Ψ).
  1. This refers to the ability of water to perform work by moving from one area to another.
  2. We say that water moves from an area of high osmotic potential to an area of low osmotic potential.
  1. The symbol for the Greek Letter Psi is the Trident (Ψ). Like Poseidon had… the Greek God of the Oceans.
  2. Water always moves High (Ψ)  Low (Ψ).
  1. High (Ψ) is referred to as Hyper-osmotic. (“hyper” means “above” or “a lot”)
  2. Low (Ψ) is referred to Hypo-osmotic. ( “Hypo” means “below” or “very little”)
  3. So water always moves Hyper-osmotic  Hypo-osmotic.
  1. Terms associated with solute concentrations in water.
  1. Hypertonic (“Hyper” means “a lot”; “tonic” means “in the water”) A lot in the water.
  1. This term refers to having “a lot” of solute dissolved in a solvent (water).
  1. Hypertonic (“Hypo” means “very little”; “tonic” means “in the water”) Little in the water.
  1. This term refers to having “very little” solute dissolved in a solvent(water).
  1. The solute always flows Hypertonic  Hypotonic.
  2. The wateralways flows Hypotonic  Hypertonic. Water always flows in the opposite direction from the solute. VERY IMPORTANT!

a.Water has more “free molecules” in the Hypotonic solution.

b.Water has less “free molecules” in the Hypertonic solution. They are “busy” with making Hydration Shells around the solute molecules.

c.So water moves from the Hypotonic  Hypertonic to try and dissolve the solute.

d.When both sides have equal amounts of dissolved solute we have achieved equilibrium.

  1. Equilibrium – when the concentrations of solute are the equal on both sides of a membrane and the movement of solute molecules AND water molecules is occurring in both directions at equal rates.
  • Remember, molecules never quit moving according to the Kinetic – Molecular Theory (Unit 5). So solute and solvent are moving constantly across the membrane but at equal rates…hence equilibrium.
  1. Calculating Osmotic/Water Potential
  1. The mathematical formula is expressed as: ΨTOTAL = ΨSOLUTE + ΨPRESSURE
  1. ΨSOLUTE Solute Potential
  1. Anytime you add solute to water, you lower the water’s ability to perform work because now the water molecules are “busy” making Hydration shells around the solute molecules.
  2. The mathematical formula for calculatingSolute Potential:

ΨSOLUTE = -iCRT

-i = Ionization constant

  • The number must always be negative as you are lowering water’s potential.
  • If the molecule does not ionize in water, such as sugar, we give it a value of: -1
  • If the molecule does ionize in water, you assign a value to represent the number of ions present.

For example, acids,bases , and salts usually have a value of 2 (possibly 3).

NaCl in water dissociates (ionizes) to Na+ AND Cl- (2 ions present)

C = Molar concentration of the solvent.

R = Ideal Gas constant = 0.0831

T = temperature in Kelvins

  1. ΨPRESSURE
  1. In terms of pressure the is:
  1. Positive Pressure - a “pushing” pressure with a positive value
  2. Negative Pressure - a “pulling or sucking” pressure with a negative value.
  1. Liquids open to the air, like in a beaker; have a pressure value of zero.
  2. Liquids such as water being pulled up through xylem of trees will have a negative value.
  3. Liquids pushing against a cell wall will have a positive value.

For example: Calculate the Total Water Potential for the following solution: The molar concentration of a

NaCl solution in an open beaker has been determined to be 0.3M. The pressure potential of a

solution open to the air is zero. What is the water potential for this example at 27 degrees C?

Step 1: Convert your Temperature to Kelvin.

27OC = 273 = 300 K

Step 2: Calculate your ionization constant (Does it ionize or not… no – 1; yes – 2 or -3)

NaCl – yes … a value of -2

Step 3: Calculate the Solute Potential.

ΨSOLUTE = -iCRT = (-2)(0.3 M)(0.0831)(300 K) = -14.96

Step 4: Determine your Pressure (Positive or Negative)

We have an open beaker = 0 pressure.

Step 5: Combine to find Total potential.

ΨTOTAL = ΨSOLUTE + ΨPRESSURE = (-14.96) + 0 = -14.96 bars of potentials

Water Potential is measured in bars.

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