Name: ___________________________
Chemistry 360L, Laboratory Examination 1
March 20, 1997
General Instructions. Read each question carefully. Show your setup for all calculation problems so I can give partial credit if necessary. Include units of measurement on all answers. If you need a calculator, pencil, eraser, more paper, etc., ask.
Answer key
1. The specific rotation of an optically active substance is given by the equation:

A student determines that the optical rotation of water in a 1 dm polarimeter tube is -2.3, -3.0 and -2.7o in three determinations. 2.16 g of an unknown monosaccharide was dissolved and diluted to 25.00 mL in deionized H2O. Immediate polarimeter readings showed the rotation to be an average of 12.3o. When equilibrium was reached the observed rotation was 7.0o. From the accompanying table, what was the form of the most likely monosaccharide?
Specific Optical Rotations [a]20D of Monosaccharides (in degrees)
Sugar a-Form Equilibrium mixture ß-Form
D-Ribose  -23.1 -23.7 -----
L-Arabinose +54.0 +104.5 +175.0
D-Xylose +92.0 +19.0 -20.0*
D-Glucose +113.4 +52.2 +19.0
D-Galactose +144.0 +80.5 +52.0
D-Fructose -21.0* -92.0 -133.5
D-Mannose +34.0 +14.6 -17.0
L-Rhamnose -7.7 +8.9 +54.0*
*Calculated value

2. Write the reaction used in the saponification of a triglyceride with KOH.
 

3. From the following data, compute the iodine number of an unknown fat. 0.29[53] g of a yellow-tinted liquid were placed into a 500 mL Ehrlenmeyer flask and dissolved in 10.0 mL chloroform. 30.00 mL of Hanus iodine were added and the reaction stirred intermittently for 30 minutes. 10.0 mL of 20% KI and 100.0 mL of deionized water were added. The initial burette reading was 0.23 mL. 0.0984 N sodium thiosulfate were added until a straw-colored solution was obtained. 15 drops of 1% starch were added and the solution was noted to be an intense blue color. After 3 more drops of thiosulfate were added, the solution turned colorless, with no change in temperature noted. The final burette reading was 42.67[35.52] mL. Titration of 10.0 mL Hanus iodine required 11.49[18.32] mL thiosulfate.

The reaction involved is: I2 + 2 Na2S2O3 ----> 2 NaI + Na2S4O6
 

4. Given the structures of cholesterol and lecithin, explain how they can be separated.
 

5. What is the relationship of saponification number to mass of a triglyceride molecule? Explain your answer.