States of Matter

Lesson 4.9

CHEMISTRY 2

HONORS

Jeff Venables

Northwestern High School

Osmosis

• Semipermeable membrane: permits passage of some

components of a solution. Example: cell membranes and

cellophane.

• Osmosis: the movement of a solvent from low solute

concentration to high solute concentration.

• There is movement in both directions across a

semipermeable membrane.

• As solvent moves across the membrane, the fluid levels

in the arms becomes uneven.

• Eventually the pressure difference between the arms

stops osmosis.

• Osmotic pressure, , is the pressure required to stop

osmosis:

• Isotonic solutions: two solutions with the same

separated by a semipermeable membrane.

iMRT

RTV

ni

inRTV

• Hypotonic solutions: a solution of lower than a

hypertonic solution.

• Osmosis is spontaneous.

• Red blood cells are surrounded by semipermeable

membranes.

• Crenation:

– red blood cells placed in hypertonic solution (relative to

intracellular solution);

– there is a lower solute concentration in the cell than the

surrounding tissue;

– osmosis occurs and water passes through the membrane out of

the cell.

– The cell shrivels up.

Osmosis

Colligative Properties

• Hemolysis:

– red blood cells placed in a hypotonic solution;

– there is a higher solute concentration in the cell;

– osmosis occurs and water moves into the cell.

– The cell bursts.

• To prevent crenation or hemolysis, IV (intravenous)

solutions must be isotonic.

– Cucumber placed in NaCl solution loses water to shrivel up and

become a pickle.

– Limp carrot placed in water becomes firm because water enters

via osmosis.

– Salty food causes retention of water and swelling of tissues

(edema).

– Water moves into plants through osmosis.

– Salt added to meat or sugar to fruit prevents bacterial infection

(a bacterium placed on the salt will lose water through osmosis

and die).

• Active transport is the movement of nutrients and waste

material through a biological system.

• Active transport is not spontaneous.

Example

What is the osmotic pressure of a solution of 7.95 g of

NaCl in 50.0 mL of an aqueous solution at 75˚C?

155 atm

118,000 mm Hg

One major application of vapor pressure lowering and

colligative properties is in molar mass problems

1. An aqueous solution contains 1.00 g/L of a detergent.

The osmotic pressure of this solution at 25˚C is 17.8 torr.

What is the molar mass of the detergent?

2. 1.008 g of a compound was dissolved in

11.38 mL of benzene (d=0.879 g/mL) and

the solution froze at 4.37˚C. What is the

molar mass of the compound?

Tf(benzene) = 5.48˚C

Kf(benzene) = 5.12˚C/molal

• Typical hydrophilic groups are polar (containing C-O, O-

H, N-H bonds) or charged.

• Hydrophobic colloids need to be stabilized in water.

• Adsorption: when something sticks to a surface we say

that it is adsorbed.

• If ions are adsorbed onto the surface of a colloid, the

colloids appears hydrophilic and is stabilized in water.

• Consider a small drop of oil in water.

• Add to the water sodium stearate.

• Sodium stearate has a long hydrophobic tail

(CH3(CH2)16-) and a small hydrophobic head (-CO2-Na+).

• The hydrophobic tail can be absorbed into the oil drop,

leaving the hydrophilic head on the surface.

• The hydrophilic heads then interact with the water and

the oil drop is stabilized in water.

Colloids

• Most dirt stains on people and clothing are oil-based.

Soaps are molecules with long hydrophobic tails and

hydrophilic heads that remove dirt by stabilizing the

colloid in water.

• Bile excretes substances like sodium stereate that forms

an emulsion with fats in our small intestine.

• Emulsifying agents help form an emulsion.