CHEM 1120 1st Edition Lecture 3 Outline of Last Lecture I Solution Process and Entropy II Gas Solubility Pressure and Temperature III Units of Concentration IV Colligative Properties Outline of Current Lecture I Continuing with Colligative Properties Freezing Point Depression II Boiling Point Elevation III Osmosis Current Lecture I i van t hoff factor 2 when a substance ionizes usually 1 Tb Tb boiling point elevation ikb m kb molal boiling point elevation constant m molality Tf ikf m Tf freezing point depression kf molal freezing point depression constant Note that the constants for each equation differ for different solvents These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Example Vinegar is essentially a 5 0 w w solution of acetic acid HC2H3O2 in water What is the freezing point of vinegar kf 3 90 acetic acid 1 86 water units degrees C m Solution Assume 1 kg of solution 0 95 kg water 0 05 kg 50 g acetic acid 50 g 60 05g mol 0 8326 mol acetic acid molality 0 8326 mol 0 95 kg 0 8764 m Tf ikf m 1 86 0 8764 1 63 degrees C II Example Seawater is about 3 5 NaCl Calculate the normal boiling point of seawater kb 0 51 Assume 100 g of solution 3 5 g NaCl 96 5 g water 3 5 g NaCl x 1 mol 58 5 g mol 0 0598 mol m 0 0598 kg 0 612 Tb ikb m 0 612 degrees C 0 64 degrees C mol 0 0965 m 2 x 0 51 x 0 64 100 106 4 III Osmosis is described as the diffusion of solvent through a semi permeable membrane into a solution of higher concentration A semi permeable membrane is a membrane that only allows certain solvent molecules to go through to the other side of the membrane but not solute molecules Some examples of osmosis are nutrient transport in plants and animals carrots regain crispness in water meat is preserved in salt cucumbers shrivel to become pickles in brine Osmotic Pressure the pressure resulting from osmosis and equal to the minimum counter pressure necessary to prevent osmosis Osmotic pressures can get very high Similar to Ideal Gas Law MRT M molarity R 0 08206 T in Kelvin Example What is the osmotic pressure of a 1 0 M sucrose solution at room temperature 20 degrees C Solution MRT 1 mol L x 0 08206 L atm mol K x 293 K 24 atm If the osmotic pressure is the same on both sides of a membrane concentrations are equal the solutions are considered isotonic If the solute concentration outside the cell is greater than that inside the cell the solution is hypertonic Water will flow out and crenation contraction of a cell after exposure to a hypertonic solution due to the loss of water through osmosis will occur If the solute concentration outside the cell is less than the solute concentration inside the cell the solution is hypotonic Water will flow into the cell and hemolysis rupturing of the cell which then releases its contents and fluids into the surrounding fluid will occur
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