This is question #2 from the 1993 AP Chemistry test with a different substance. Only a partial answer is supplied. I put it at the end of the file.

Elemental analysis of an unknown pure substance indicates that the percent composition by mass is as follows:

Carbon - 42.87%
Hydrogen - 2.40%
Nitrogen - 16.66%
Oxygen - 38.07%

A solution that is prepared by dissolving 3.152 grams of the substance in 25.00 grams of benzene, C₆H₆, has a freezing point of 1.66°C. (The normal freezing point of benzene is 5.50°C and the molal freezing-point depression constant, K_f, for benzene is 5.12 C°/molal.)

(a) Determine the empirical formula of the unknown substance.

(b) Using the data gathered from the freezing point depression method, calculate the molar mass of the unknown substance.

(c) Calculate the mole fraction of benzene in the solution described above.

(d) The vapor pressure of pure benzene at 35°C is 150. millimeters of Hg. Calculate the vapor pressure of benzene over the solution described above at 35°C.

dinitrobenzene (C₆H₄N₂O₄) mol. wt. = 168.1 with emp. for. = C₃H₂NO₂