An Erlenmeyer flask that weighs 93.667g has a quantity of liquid vaporized in it
ID: 909761 • Letter: A
Question
An Erlenmeyer flask that weighs 93.667g has a quantity of liquid vaporized in it at 96.4oC and a pressure of 1583.8 torr. The mass of the cooled flask is 94.345g. When the flask is filled with water, the mass of the flask and water is 268.92g.
(a). Determine the molecular weight of the unknown liquid. ____________ g/mol ** Write answer in decimal form to one decimal place, NO units in answer**
(b). Given the following compounds:
Liquid Molecular Weight (g/mole)
Pentane 72.2
Acetone 58.1
Methanol 32.0
Hexane 86.2
Ethanol 46.1
Propanol 60.1
What is the identity of the unknown liquid?
Explanation / Answer
Pressure of the vaporized liquid = 1583.8 torr = 2.084 atm (1 torr = 0.00131579 atm)
Temperature = 96.4 C = 369.4 K
mass of the vapour = condensed vapour mass = 94.345g - 93.667 g = 0.678 g
Mass of the flask = 93.667g - 0.678g = 92.9887 g
The mass of the flask and water is 268.92g, Hence mass of the water = 268.92-92.9887 = 175.9313 g
Volume of the flask = weight of the water / density = 175.9313 / 0.9982 = 176.2 ml
Let us use the gas equation PV = nRT
PV = (w/MW) x RT
MW = (w x R x T) / (P x V) = (0.678g x 0.0821 L.atm/mole.K x 369.4 K) / (2.084 atm x 0.1762 L)
The molecular weight of the unknown liquid = 55.99 g/mole
Acetone with molecular weight 58.1g/mole is the identity