Experimental demonstration 4. Energy Crisis – Chem 021
Measuring molecular weight (molar mass)
A small amount of liquid (e.g. methanol or ethanol) will be evaporated by heating a flask.
Excess gas ill exit the container through a small pinhole punched on the aluminum foil cover held in place by a rubber band.
At the moment when all the liquid has vaporized including the droplets on the neck of the flask, lift the flask and set it down to cool. By measuring the volume of the gas (flask) and the weight of the condensed liquid, we can calculate the molar mass (M):
M= m*24.2 L/V [amu/g], (equation 1) where V is the volume of the flask in L, and m is the extra weight in grams of the droplet.
Let us estimate the amount of liquid needed, if the liquid is methanol, CH
3
OH:
M=1*12+1*16+4*1=32 amu. (Actually 32.04, why the difference?)
From equation 1: m= 32 amu *V/24.1L * g/amu. If V=125 mL = 0.125 L
 m=0.166g = 166 mg.
We can easily measure the volume of a liquid, how much volume does 166 mg take up? d(ensity)= m/V. Density of ethanol is 0.791 g/mL

V= m/d = 0.21 mL. We need more for evaporation, 1-2 mL will suffice.
Apparatus and parts:
Hot plate, container for heat bath.
Scale to measure the mass of the flask with rubber band and Al foil first (m i
)and then mass of the flask with rubber band and Al foil and the condensed gas (m f
). The difference of the two is the mass of the condensed gas.
Measure the volume of the flask with water and another graduated cylinder .
Measure the temperature of the water bath with a thermometer .
Supplies: needle, paper towel, tongue, goggle, liquid (known or unknown) .
Safety instructions and concerns. Please read carefully and follow :
Accidents happen because of carelessness. Pay attention to the experiment, what you are doing, and to safety.
Some of the liquids we deal with, especially methanol, are poisons . Do not ingest!
Hot plates are hot , do not touch, do not make any sudden moves. Do not spill the heating bath, which is boiling water. Use goggles from the very start of any experimentation.
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Procedure for measurement:
1.
Heat water for the heating bath.
Ask for approval before starting this step.
2.
Measure the volume of your flask, including the neck.
3.
Measure the initial mass, m i
as defined above.
4.
Measure the temperature of the heat bath.
5.
Add a small but sufficient amount of liquid in your flask, about 2 mL.
6.
Seal, puncture the top with a pin to create the pinhole. The hole should be small so that the loss of gas can be sufficiently small.
7.
Hold the flask with the liquid inside and sealed in the heat bath with a tongue.
This should be done carefully. Ask for approval before starting this step.
8.
Perform the evaporation, checking the presence of liquid at the bottom and neck of the flask
9.
Stop when all liquid is gone, set down to cool, dry the outside of the flask.
10.
Measure the final mass, m f
as defined above.
11.
Calculate m= m f
- m i
, from it, and V calculate M.
12.
Analyze data, make possible corrections.
13.
Put together all data obtained by the different teams of the whole class in order to perform a statistical analysis for calculating the average observed value, reject possible outliers, and the estimate the error of the observed quantity.
Possible corrections and refinements: the gas is not at “normal temperature and pressure” and so the volume of 1 mol would not be 24.1 L, but:
V(corrected)=24.1L*1 atm* 294K/pT, where p and T are the actual pressure (p, in atm) and temperature, (T, in Kelvin), respectively.
A few basic data from the Handbook of Chemistry and Physics, 63 rd Edition, CRC Press, 1983.: d Bp (in C) M page methanol ethanol octane
0.791
0.789
114.23
65.15
78.5
125.7
32.04
46.07
.783
C-374
C-290.
C-404
A few questions about this experiment:
-Was there a chemical reaction or chemical transformation involved in this experiment?
-This experiment does not focus on energy. Nevertheless, energy is involved in various steps. Which forms of energy are involved in this experiment, and can you list which forms are transform into which other forms in the various steps, if any.
-How many mL of octane would you need at a minimum to do this experiment for octane? Would the experiment work, if not, why? How would you have to modify the experiment to make it work?
© Copyright, 2004, Miklos Kertesz (Revised 9-28-04)
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