Molar Mass of a Volatile Liquid
Introduction
It is often useful to know the molecular mass of a substance since it is one of the properties that help characterize the substance. If the substance is a volatile liquid, one common way of determining its molecular mass involves using the ideal gas law, PV = nRT.
Since the liquid is volatile, it can easily be converted to a gas. While it is in the gas phase, its volume, temperature and pressure are measured. The ideal gas law will then allow the calculation of the number of moles of the substance present.
The mass of the gas is found by cooling the gas so that it condenses back into a liquid and then determining the mass of the condensed liquid. Molecular mass can be calculated from the number of moles and the mass of the sample.
Key Relationship
PV = nRT
P from room barometer
V from water your flask holds. Measure this carefully.
R= 0.08206 L(Atm)/mol.(K)
T measured using your calibrated thermometer. Use your calibration curve to correct the apparent temperature to the true temperature.
Key Relationship
PV = nRT
solve for n.
Then
n = mass/GFW
solve for GFW
Procedure
Weigh clean, dry 250 mL Erlenmeyer flask with Aluminum foil and rubber band.
Place 3-5 mL of Unknown in flask.
Seal with Aluminum foil and rubber band.
Secure with utility clamp in 600 mL beaker with at least 200 mL water.
Poke a small hole in foil with pin. Heat slowly to boiling.
Turn heat down to maintain slow boil
Watch liquid as it evaporates. Continue heating 1 minute after all visible liquid vaporizes.
Measure the temperature of the water bath. This, presumably equals the temperature of the flask.
Record this data.
At or about the same time, record the atmospheric pressure of the room.
Quickly cool the flask to room temp. Use the room temperature water bath. The liquid vapor will condense, this is OK.
Dry your flask completely and re-weigh.
Don't waste unnecessary time at this step.
Try to maintain 5 sig. fig. with masses.
A minimum of 3 separate determinations need to be made of your unknowns.
All steps and data are to be recorded in pen in your lab notebook.
Example (with cleverly faked data) ; )
Lab 3: Molar Mass of Liq.
Date: 10/2/07
Lab partners: Isaac N, J. Priestly, J. Dalton
Unknown D
Trial 1:
Mass of empty dry flask, Al, and band: 131.168 g
Temp of boiling water
Raw = 96.3 C Corrected 94.3 C = 367.3K
Barometric pressure 858 hPA
858 hPA/1013.25 hPA/1 atm =0.8468 atm
Mass of flask + vapor = 131.378
Net mass of vapor = 131.378 - 131.168 = 0.210 g
Volume of flask 274.1 mL = 0.2741 L
Calculations
PV=nRT
n = RT/PV
n = (0.8468) (0.2741)
(0.08206) (367.3)
n = 7.700 x 10-3 moles
GFW = m/n
GFW = 0.210 g/ 7.700 x 10-3 moles
GFW = 27.27 g/mole
Trial 2
Trial 3
Average GFW
Identify 3 sources of error or potential error
Speculation of what organic liquid this could be.
50 points of extra credit will be available for each additional unknown successfully determined. Maximum of 2.