A certain volatile hydrocarbon (a binary compound of carbon and hydrogen) is fou

Question

A certain volatile hydrocarbon (a binary compound of carbon and hydrogen) is found to be 92.3 % carbon, by mass. In a separate experiment, utilizing the Dumas method, a 4.00 mL pure liquid sample of this hydrocarbon is vaporized in a 125 mL Florence flask when the barometric pressure is 768.0 torr. After the excess gas escapes, the temperature is measured as 98.0 degree C. The flask and contents are subsequently cooled to 25 degree C (density water at 25 degree C = 0.997044 g/mL) and the vapor condenses to a liquid. The flask is then emptied, cleaned, and filled with water. When weighed on a balance, the difference in weight between the flask filled to the brain with water and the dry empty flask at 25 degree C is 128.12 g. The empty flask - fitted with a foil cap pierced with a pinhole - weighs 25.3478 g The weight of the flask and contents is found to be 25.6803 g Please determine the following, The empirical formula of this hydrocarbon. The volume that the vapor occupied in the Erlenmeyer flask (in L). The molecular formula of this hydrocarbon.

Explanation / Answer

Molecular formula determination

moles (n) = PV/RT

                 = (768/760) x 0.125/0.08205 x (273 + 98)

                 = 0.00415 mol

molar mass of hydrocarbon = (25.6803 - 25.3478)g/0.00415 mol

                                             = 80.13 g/mol

Carbon = 0.923 x 80.13 = 73.96 g

moles of C = 73.96/12 = 6.16 mol

rest H = 6.17 g

moles of H = 6.17/1 = 6.17 mol

Empirical formula = CH

empirical formula mass = 13 g

Volume vapor occupied = 0.125 L

ratio = 80/13 = 6

Molecular formula = C6H6

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