A sample of an unknown volatile liquid was injected into a Dumas flask m_flask = 27.0928 g, v_flask = 0.1040 L)and heated until no visible traces of the liquid could be found. The flask and its contents were then rapidly cooled and reweighed (m_{flask + vapor} = 27.4593 g). The atmospheric pressure and temperature during the experiment were 0.976 atm and 18.0 °C, respectively. The unknown volatile liquid was ________.

Question

Quizplus · Accepted Answer

To determine the identity of the unknown volatile liquid, we can use the Dumas method which involves measuring the mass of the substance and its vapor at a known temperature and pressure. Using the data provided, we can determine the number of moles of the substance and use this information to identify it.
First, we need to calculate the moles of gas using the ideal gas law:
PV = nRT
n = (PV) / RT
n = (0.976 atm) (0.1040 L) / [(0.08206 L*atm/(K*mol))(18.0°C + 273.15)]
n = 0.00404 mol
Next, we can calculate the molecular weight of the unknown volatile liquid using its mass and the moles of gas:
MW = m_gas / n
MW = (27.4593 g - 27.0928 g) / 0.00404 mol
MW = 92.2 g/mol
Looking at the answer choices, the only substance with a molecular weight close to 92.2 g/mol is choice (B), C6H14.

A Sample of an Unknown Volatile Liquid Was Injected into a Dumas