Chapter 12 Solutions and Osmosis Dr. Peter Warburton

© Peter Warburton 2008 All media copyright of their respective owners2 Pure substances We’ve seen that the physical properties of a pure substance are dependent on the strength of the intermolecular forces between the molecules

© Peter Warburton 2008 All media copyright of their respective owners3 Solutions Solutions are “mixtures” of two or more pure substances. The substance found in greater amounts is usually referred to as the SOLVENT, while all other substances in the solution are considered to be the SOLUTE(S).

© Peter Warburton 2008 All media copyright of their respective owners4 Solutions Solutes and solvents only tend to mix (are MISCIBLE) when the intermolecular forces of the solvent and solute(s) are of similar strengths. “Like dissolves like”

© Peter Warburton 2008 All media copyright of their respective owners5 Figure 12.5 If molecules A and B have large differences in intermolecular force strengths, then a molecule of A will be “sucked back” into pure A instead of mixing with pure B. Oil (London forces) and water (hydrogen bonds) DON’T MIX, and are said to be IMMISCIBLE.

© Peter Warburton 2008 All media copyright of their respective owners6 Figure 12.6 If molecules A and B have similar intermolecular force strengths, then a molecule of A can mix with pure B. Alcohols (hydrogen bonds) and water (hydrogen bonds) DO MIX!

© Peter Warburton 2008 All media copyright of their respective owners7 Physical properties of solutions Even though “like dissolves like” the presence of solute molecules in the solvent disrupts the bulk solvent intermolecular forces to some extent. This change in forces means that the physical properties of solutions, like freezing and boiling points, are slightly different than those of the pure solvent.

© Peter Warburton 2008 All media copyright of their respective owners8 Physical properties of solutions The disruption of forces generally depends more on the amount of solute (in terms of concentration) rather than the chemical identity of the solute. Such colligative properties of solutions therefore depend on the concentration of the solution while ignoring the identity of the solute.

© Peter Warburton 2008 All media copyright of their respective owners9 Dilution of solutions If we take a solution and add pure solvent to it, we expect dilution to occur. The solution and solvent mix until a new solution of lower concentration is made.

© Peter Warburton 2008 All media copyright of their respective owners10 Dilution of solutions If we instead take a solution and separate it from pure solvent using a semipermeable membrane that solute molecules can’t pass through, then we are “controlling the mixing.”

© Peter Warburton 2008 All media copyright of their respective owners11

© Peter Warburton 2008 All media copyright of their respective owners12 Osmosis Osmosis is the process where the solvent passes through the semipermeable membrane to EQUALIZE the solute concentration on both sides of the membrane.

© Peter Warburton 2008 All media copyright of their respective owners13

© Peter Warburton 2008 All media copyright of their respective owners14 Osmotic pressure Osmotic pressure is a colligative property of a solution that is defined as the external pressure that must be applied to a solution to just stop the process of osmosis through the membrane.

© Peter Warburton 2008 All media copyright of their respective owners15 Osmotic pressure Osmotic pressure  depends on the concentration of the solution (M), the temperature (T) and the gas constant (R).  = MRT

© Peter Warburton 2008 All media copyright of their respective owners16 Osmotic pressure and molar mass 1.Make a solution of a certain mass of solute in a given total volume. 2.Measure the osmotic pressure. 3.Calculate M from measured  and T. 4.Use M and volume to calculate moles of solute. 5.Use moles and mass of solute to calculate molar mass.

© Peter Warburton 2008 All media copyright of their respective owners17 Problem An aqueous solution of 21.6 mg of vasopressin in mL of solution has an osmotic pressure at 25  C of 3.70 mmHg. What is the molar mass of the hormone?

© Peter Warburton 2008 All media copyright of their respective owners18 Osmosis and biology Cell walls are often semipermeable membranes that allow water to pass through, but not many solute molecules. Body fluids, like blood plasma, must have concentrations that match those inside the cell or osmosis might cause cells to shrink or rupture.

© Peter Warburton 2008 All media copyright of their respective owners19 Blood cells and osmosis Normal blood cell in isotonic (same concentration) solution. No osmosis. Collapsed blood cell in hypertonic (more concentrated) solution. Water leaves the cell during osmosis. Bloated blood cell in hypotonic (less concentrated) solution. Water enters the cell during osmosis.