RooneyMWLab
Molecular Weight Experiment
Mary Anne Rooney
November 11, 2006
MISE - Physical Basis of Chemistry
Third Experiment
Molecular Weight of a Volatile Liquid
Lab Report – Submit electronically (digital drop box) by Sunday, November 18, 6:01PM
Note: When submitting to digital Drop Box label you files with your name first and then a
brief description of what the document is.
Purpose: The Purpose of this experiment is to determine the molecular weight of a compound
that is a volatile liquid.
DATA/CALCULATIONS
Note: Data and Calculations may be completed on an Excel spreadsheet.
Unknown Letter: B
Molecular Weight of a Volatile Liquid
(If you performed more than one trial, adjust your data table accordingly.)
1. Determination of mass of vapor:
Weight of flask + foil cap + rubber band
Temperature of water bath when (excess) vapor ceased to
escape
Weight of flask + foil cap + rubber band + condensed
vapor
Weight of condensed vapor
Trial 1
Trial 2
91.8751 g
93.7 oC
91.8751 g
93.8 oC
92.4380 g
92.3002 g
.5629 g
.5629 g
233.14 g
91.42 g
141.72 g
23.1 oC
0.142 L
233.14 g
91.42 g
141.72 g
23.1 oC
0.142 L
Barometric pressure : 760 mmHg = 1 atmHg
770.9 mm
1.01 atm
therefore, 770.9Hg = 1.01 atmHg
Temperature (Kelvin): oC + 273 = oK
therefore, 23.1 oC + 273 = 296.1 oK
23.1 oC
296.1 oK
2. Determination of full volume of flask:
Weight of Erlenmeyer flask filled with water
Weight of clean, dry, and empty Erlenmeyer flask
Weight of water that completely filled Erlenmeyer flask
Temperature of water in the filled Erlenmeyer flask
Calculated volume of Erlenmeyer flask (in liters)
Show work here: 1.0 gH2O = 1.0 mLH2O
1000 mL = 1 L; so 141.72 mL ÷ 1000 = 0.142 L
RooneyMWLab
Molecular Weight Experiment
Mary Anne Rooney
November 11, 2006
Determination of molecular weight of the volatile liquid:
(Assuming ideal gas behavior, use the pressure of the vapor (P), its volume (V), and its
temperature (T) to determine the moles of vapor in the Erlenmeyer flask. Then, using the mass
of vapor which occupied the Erlenmeyer flask, determine the molecular weight (MW in g/mole)
of the volatile liquid. If you performed more than one trial, show all work for each trial and then
compute the average molecular weight.) Show work below.
T = 93.7 OC + 273 = 366.7 OK
MW = gRT  MW = 0.5629 g x 0.0821 atm*L x 366.7 K  16.95  118.2
PV
1.01 atm x
n * K x .142 L
.14342
g/n
(Average) Molecular Weight
g/mole
118.2
Determination of Empirical and Molecular Formula of volatile liquid:
The table below lists the pure volatile liquids that were distributed in lab. The first column gives
the letter designation of the compound and the remaining columns give the appropriate elemental
mass percents. Using the data for your particular volatile liquid, please determine the empirical
formula and the molecular formula for your particular sample. Show all work.
Sample Letter
mass % carbon (C)
mass % hydrogen (H)
mass % oxygen (O)
“A” (cyclohexane)
85.60 %
14.40 %
None
“B” (ethyl acetate)
54.52 %
9.17 %
36.31 %
“C” (2-propanol)
59.94 %
13.44 %
26.62 %
moles C = (54.52 g C) x ( 1 mole C ) = 54.52 g
(12.01 g C)
12.01 g
moles H = (9.17 g H) x (1 mole H ) =
1.01 g H
9.17 g
1.01 g
= 4.54 moles C
= 9.08 moles H
moles O = (36.31 g O) x (1 mole O ) = 36.31 g = 2.27 moles O
16.00 g O
16.00 g
moles C = 4.54 moles = 1 = CH2
moles H
9.08 moles
2
moles C = 4.54 moles = 2 = C2O
moles O
2.27 moles
1
moles H = 9.08 moles = 4 = H4O
moles O
2.27 moles
1
Empirical Formula
C2H4O
Volume vapor = Volume water = mass water
densitywater
T = 93.7
OC
Volume = 0.142 L
+ 273 = 366.7 OK
 141.72 g
 142.1 mL x 1L =
0.9975 g/mL
1000 mL
RooneyMWLab
Molecular Weight Experiment
MW = gRT
PV
Mary Anne Rooney
November 11, 2006
 MW = 0.5629 g x 0.0821 atm*L x 366.7 K  16.95  118.2
1.01 atm x
n * K x .142 L
.14342
g/n
Molecular Weight = 118.2 g/mole
EWF = (2 x 12.01g/mole C) + (4 x 1.01 g/mole H) + 16.00 g/mole O = 44.06 g/mole
N = MW =
EWF
118.2 g/mole
44.06 g/mole
= 2.650  2  Molecular Formula = C4H8O2
CONCLUSION QUESTIONS
1. Why is the barometric (i.e., atmospheric) pressure considered to be the pressure of the vapor, i.e.,
how does the experimental procedure ensure this? Explain carefully. The barometric pressure of a
substance in a non-porous container that has a pin hole in the “lid” will balance out to
be the same inside the container as is the barometric pressure outside the container.
2. Why isn't it necessary to weigh the amount of liquid initially put into the flask? Explain.
It isn’t necessary to weigh the amount of liquid initially put into the flask because we
were concerned with the molecular weight of the vapor, not the liquid. By heating the
liquid until vaporized, we were permitting the container to fill completely with vapor –
vapor (gas) will expand to fill the available space.
Extra Credit–
It is sometimes stated that the above method (Dumas method) relies on the presumption that the
investigated gas follows the ideal gas law (PV = nRT). This allows one to determine the moles
of the contained vapor via:
PV
n = moles of contained vapor = RT , i.e., its measured pressure (P), volume (V), and
temperature (T). Of course, the gas constant (R) is tabulated and equals 0.0821 L•atm•mol-1•K-1.
Then, the molecular weight of the gas (MW in g/mol) can be determined from the measured
mass of the contained vapor (g in grams) divided by the calculated number of moles.
g
g
gRT
MW = n = PV =
. In other words, if the gas did not behave ideally, then the
PV
 
RT
 
calculated molecular weight would be a crude approximation at best. Please explain why this is
commonly not a big problem, i.e., why an approximate value of the molecular weight is often
sufficient when determining the actual chemical formula of a volatile compound. What other
information is commonly obtained for a compound in the process of determining its chemical
formula such that an approximate molecular weight is often “good enough”?
An approximate value of the molecular weight is often sufficient when
determining the actual chemical formula of a volatile compound because the law
of multiple proportions allows us to obtain the empirical formula equal to the
sums of the atomic weights of the constituent elements.