To determine the molar mass of a volatile liquid by the Dumas method, a student adds ~2 mL of unknown liquid to a flask sealed with foil pierced by a pinhole. Mass of empty flask + foil = 71.250 g. The flask is heated in a 100.0 C water bath until all liquid is vaporized and excess vapor escapes. After cooling, the flask is wiped dry and reweighed: 72.105 g. Flask volume = 248.0 mL; atmospheric pressure = 1.00 atm.
What is the molar mass of the unknown liquid?
- Acheck_circle
106 g/mol
- B
232 g/mol
- C
32 g/mol
- D
85 g/mol
Explanation
Mass of vapor = 0.855 g. n = PV/RT = (1.00)(0.2480) / (0.08206 * 373.2) = 8.10e-3 mol. M = 0.855/8.10e-3 = 106 g/mol.