An Erlenmeyer flask that weighs 93.667g has a quantity of liquid vaporized in it
ID: 904048 • 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
Solution :-
Mass of the flask = 93.667 g
Mass of cooled flask = 94.345 g
Mass of flask filled with water = 268.92 g
Mass of water = 268.92 g – 93.667 g = 175.25 g
So the volume of flask = 175.25 ml = 0.17525 L
Because density of water is 1 g/ml
Mass of liquid = 94.345 g – 93.667 g = 0.657 g
Now lets convert pressure to atm
1583.8 torr * 1 atm / 760 torr = 2.084 atm
Temperature = 96.4 C +273 = 369.4 K
Now lets calculate the moles of the vapor in the flask
PV= nRT
n= PV/RT
n= 2.084 atm * 0.17525 L / 0.08206 L atm per mol K * 369.4 K
n= 0.012048 mol
now lets calculate the molar mass of the liquid using the moles and mass
molar mass = mass / moles
= 0.657 g/0.012048 mol
= 54.5 g per mol
So the closest molar mass is the acetone = 58.1 g per mol
So the identity of the compound is acetone