Chemistry 241 Exam 2 Name ___________________________________

Summer 1998 100 pts

Lab TA _____________________________

Substances A and B are two miscible liquids that behave as an ideal mixture. Use the following liquid-vapor phase diagram for a mixture of A and B to answer questions 1 - 4. The diagram is for atmospheric pressure = 760 torr.

1. (6 pts) What are the boiling points of the pure liquids A and B ?

bpt. A = ____________ ûC

bpt B = ____________ ûC

2. (4 pts) What is the boiling point of a solution that is a mixture of 37.5 mol% A and 62.5 mol% B?

bpt. mixture = ____________ ûC

3. (4 pts) What is the composition of the vapor in equilibrium with a solution that is a mixture of 37.5 mol% A and 62.5 mol% B?

mol% A (vapor) = ___________

4. (6 pts) A solution that is about 85 mol% B is fractionally distilled with a distillation column that has an efficiency of two theoretical plates. Estimate the mol% of A that would be obtained in the first few drops of distillate from the fractional distillation.

mol% A (distillate) = ___________

5. (6 pts) Acetone and chloroform are two miscible liquids. The chloroform hydrogen is relatively acidic and will hydrogen bond with the acetone carbonyl. Is Raoult’s law likely to hold true for a solution of acetone and chloroform ? Explain your answer.

Use the following gas chromatograph of a mixture of dioxane (bpt = 101 ûC) and cyclohexane (bpt = 81 ûC) eluted on a non-polar column to answer questions 6 - 8.

6. (6 pts) What is the identity of the substance labeled peak A? Explain how you deduced this.

peak A = ____________________

explanation:

7. (6 pts) The area measured for peak A is 2 cm² and the area measured for peak B is 3 cm². Assuming that the detector response is the same for both substances, calculate the mol% of A in the mixture.

mol% A = ___________

8. (6 pts) A student was given a mixture of dioxane and cyclohexane. The student did a fractional distillation of the mixture and did gas chromatography of the first few drops of the distillate and of the original mixture. In one chromatograph the areas of peaks were A = 4 cm² and B = 1 cm². In the second chromatograph the areas of peaks A = 1 cm² and B = 3 cm². Which of the two chromatographs corresponds to the original mixture ?

Consider the boiling points of the following solvents to answer questions 9 and 10.

9. (4 pts) Which of the above solvents has the lowest vapor pressure at room temperature?

10. (4 pts) Which of the above solvents is the most volatile ?

11. (8 pts) Substances C and D are two miscible liquids that behave as an ideal mixture. The vapor pressure of pure C is 920 torr at 80 ûC. A solution composed of 50 mol% C and 50 mol% D boils at 80 ûC at atmospheric pressure (760 torr). Calculate the vapor pressure of pure D at 80 ûC.

12. (8 pts) A mixture of the four compounds shown below is separated by silica gel TLC to give four spots with the following Rf values: 0.12, 0.44, 0.82 and 0.95. Indicate which compound corresponds to each spot on the TLC (label each spot with the Rf value).

PhCO₂CH₃ PhCH₃ PhCH₂NH₂ PhOCH₃

13. (8 pts) You are given a sample composed of three compounds, A-C with different polarities. The sample is loaded onto a silica column and eluted with dichloromethane. After eluting with 10 mL of solvent and collecting 1 mL fractions, compounds A and B are found to be in the 3rd and 6th fractions, respectively.

(a) Which compound is more polar, A or B?

(b) Is compound C more or less polar than compounds A & B?

(c) After eluting with another 10 mL of dichloromethane, compound C has still not eluted. Describe how you would obtain compound C.

14. (12 pts) An electrophoresis gel of the compounds A-D is shown below. The gel is highly cross-linked with small pore sizes. Label the gel to indicate which end is the cathode and anode and label each spot to indicate its identity as compound A-D.

15. (12 pts) Each set of hydrogens on the molecule below are labeled with a letter. Use these letters to designate which NMR resonances in the spectrum correspond to the protons in the molecule.