Solutions & Spectrophotometry Colligative Properties

Osmosis Some substances form semipermeable membranes, allowing some smaller particles to pass through, but blocking other larger particles. In biological systems, most semipermeable membranes allow water to pass through, but solutes are not free to do so.

Osmosis In osmosis, there is net movement of solvent from the area of higher solvent concentration (lower solute concentration) to the are of lower solvent concentration (higher solute concentration).

Osmotic Pressure n  = ( )RT = MRT V The pressure required to stop osmosis, known as osmotic pressure, , is n V  = ( )RT = MRT where M is the molarity of the solution If the osmotic pressure is the same on both sides of a membrane (i.e., the concentrations are the same), the solutions are isotonic.

SAMPLE EXERCISE 13.11 Calculations Involving Osmotic Pressure The average osmotic pressure of blood is 7.7 atm at 25°C. What concentration of glucose (C6H12O6) will be isotonic with blood? Solve: Comment: In clinical situations the concentrations of solutions are generally expressed as mass percentages. The mass percentage of a 0.31 M solution of glucose is 5.3%. The concentration of NaCl that is isotonic with blood is 0.16 M because NaCl ionizes to form two particles, Na+ and Cl– (a 0.155 M solution of NaCl is 0.310 M in particles). A 0.16 M solution of NaCl is 0.9 mass % in NaCl. This kind of solution is known as a physiological saline solution.

Osmosis in Blood Cells If the solute concentration outside the cell is greater than that inside the cell, the solution is hypertonic. Water will flow out of the cell, and crenation results.

Osmosis in Cells If the solute concentration outside the cell is less than that inside the cell, the solution is hypotonic. Water will flow into the cell, and hemolysis results.

Molar Mass from Colligative Properties We can use the effects of a colligative property such as osmotic pressure to determine the molar mass of a compound.

Predict which aqueous solution will have the lowest freezing point. 0.25 m C2H5OH 0.15 m CaCl2 0.20 m NaCl 0.15 m NH4NO3 0.15 m Na3PO4

Predict which aqueous solution will have the lowest freezing point. 0.25 m C2H5OH 0.15 m CaCl2 0.20 m NaCl 0.15 m NH4NO3 0.15 m Na3PO4

Arrange the aqueous solutions according to increasing boiling point. AlCl3 < KNO3 < Na2SO4 Na2SO4 < AlCl3 < KNO3 Na2SO4 < KNO3 < AlCl3 KNO3 < AlCl3 < Na2SO4 KNO3 < Na2SO4 < AlCl3 0.10 m Na2SO4 0.15 m AlCl3 0.20 m KNO3

Arrange the aqueous solutions according to increasing boiling point. AlCl3 < KNO3 < Na2SO4 Na2SO4 < AlCl3 < KNO3 Na2SO4 < KNO3 < AlCl3 KNO3 < AlCl3 < Na2SO4 KNO3 < Na2SO4 < AlCl3 0.10 m Na2SO4 0.15 m AlCl3 0.20 m KNO3

Spectrophotometric Determination [Co2+] Spec 20 Lab Spectrophotometric Determination [Co2+] 13

Machine that measures amount of light transmitted through a solution Spectrophotometer Machine that measures amount of light transmitted through a solution 14

Calibration of Spec 20 Setting zero Setting 100% 15

Spec 20’s Zero Newer Spectrophotometer: 1. Without anything inside, press zero set button 2. Adjust left knob so scale reads exactly zero 3. Unpress zero set button Zero Older Spectrophotometer 1. Without anything inside, adjust left knob so scale reads exactly zero 16

Set 100% (no color) Open door and place inside securely Using cuvette (small test tube) of distilled water, wipe off fingerprints with kim-wipe Open door and place inside securely Close door gently. Adjust 100% knob exactly. Remove “blank”, return to beaker. (Do not change wavelength from 620 nm) ***Repeat each time you measure on the spectrophotometer. Use same machine for entire lab! 17

Your Data: Clean and dry your cuvette Fill with 0.150 M CoCl2 to level of label Wipe fingerprints Read % T to nearest 0.1% Each time you change wavelength (25 nm) recalibrate! Graph to find optimum wavelength. 18

Finding optimum wavelength Absorbance Wavelength (nm)

Part 2] Beer’s Law Follow procedure at determined wavelength to make standardized graph.

Part 3 Finding concentration of unknown Since it is darkly colored, you will need to perform dilutions Find measurement between 0.3 and 0.5 Make sure you have at least 3 mL sample Calculations!