untitled folder 4/.DS_Store __MACOSX/untitled folder 4/._.DS_Store untitled folder 4/aaa.docx Molecular Weight of an Ideal Gas by the Dumas Method Objectives: 1. In this experiment we will determine the molecular weight of air and CO2 by measuring P, T, V and weight of a gas sample. 2. You will become familiar with how experimental errors in several measurements combine to give the error in an overall calculated result. 3. You will also become familiar with the routine operation of a vacuum rack. 4. You will refine your lab report writing skills. Introduction: In this first experiment, you will determine the molecular weight of a gas by the Dumas method. In the Dumas method, the density of a gas or volatile liquid is determined at a known pressure and temperature. Using the ideal gas law, the molecular mass of the substance can be calculated: where d is density, R is gas constant, T is temperature, P is pressure and M is molar mass of the gas. To determine the density of the gas, both mass and volume will be measured independently. A glass bulb is evacuated and filled with the test gas. The difference in the mass of the filled vs. evacuated bulb will give you the mass of the gas. The volume of the bulb is determined by measuring the amount of water required to fill the bulb. Buoyancy Correction: In this experiment we are measuring the mass of a small volume of gas by subtracting the weights of two heavy objects, an evacuated sample bulb and one filled with a gas. Changes in the temperature/density of air during the course of the experiment can result in a large amount of error in your result. To prevent this, you may need to perform a buoyancy correction to your masses. When you measure the mass of your sample bulb, you will also measure the mass of a ballast bulb. The ballast bulb is a similarly sized vessel that should have a constant mass throughout the course of the experiment. If its mass changes, then we know that the room temperature/pressure has change and we need to make a buoyancy correction. The buoyancy correction is simply, mass of Ballast Bulb (initial) – mass of the ballast bulb (final) To correct our sample mass, we subtract the buoyancy correction from our sample mass. For example, if the mass of the ballast bulb has increased by 0.2 g, we will subtract 0.2 g from the final mass of our sample bulb. Safety Concerns: 1. Safety goggles should be worn at all times. 2. The vacuum line is equipped with a mercury manometer. When filling the bulbs with CO2, caution must be taken not to have pressure above 1 atm. 3. When handling the sample bulb, carefully carry so that you don’t drop or break it. 4. The experiment requires the use of gases contained in cylinders equipped with a regulating valve. The cylinder must be securely strapped at all times. Consult your instructor on proper use of a gas regulator. Procedures: Throughout this experiment you should record the uncertainty (or notes so that you can determine ...

1. untitled folder 4/.DS_Store
__MACOSX/untitled folder 4/._.DS_Store
untitled folder 4/aaa.docx
Molecular Weight of an Ideal Gas by the Dumas Method
Objectives:
1. In this experiment we will determine the molecular weight of
air and CO2 by measuring P, T, V and weight of a gas sample.
2. You will become familiar with how experimental errors in
several measurements combine to give the error in an overall
calculated result.
3. You will also become familiar with the routine operation of a
vacuum rack.
4. You will refine your lab report writing skills.
Introduction:
In this first experiment, you will determine the molecular
weight of a gas by the Dumas method. In the Dumas method, the
density of a gas or volatile liquid is determined at a known
pressure and temperature. Using the ideal gas law, the
molecular mass of the substance can be calculated:
where d is density, R is gas constant, T is temperature, P is

2. pressure and M is molar mass of the gas.
To determine the density of the gas, both mass and volume will
be measured independently. A glass bulb is evacuated and filled
with the test gas. The difference in the mass of the filled vs.
evacuated bulb will give you the mass of the gas. The volume of
the bulb is determined by measuring the amount of water
required to fill the bulb.
Buoyancy Correction:
In this experiment we are measuring the mass of a small volume
of gas by subtracting the weights of two heavy objects, an
evacuated sample bulb and one filled with a gas. Changes in the
temperature/density of air during the course of the experiment
can result in a large amount of error in your result. To prevent
this, you may need to perform a buoyancy correction to your
masses. When you measure the mass of your sample bulb, you
will also measure the mass of a ballast bulb. The ballast bulb is
a similarly sized vessel that should have a constant mass
throughout the course of the experiment. If its mass changes,
then we know that the room temperature/pressure has change
and we need to make a buoyancy correction. The buoyancy
correction is simply,
mass of Ballast Bulb (initial) – mass of the ballast bulb (final)
To correct our sample mass, we subtract the buoyancy
correction from our sample mass. For example, if the mass of
the ballast bulb has increased by 0.2 g, we will subtract 0.2 g
from the final mass of our sample bulb.
Safety Concerns:
1. Safety goggles should be worn at all times.
2. The vacuum line is equipped with a mercury manometer.
When filling the bulbs with CO2, caution must be taken not to
have pressure above 1 atm.

3. 3. When handling the sample bulb, carefully carry so that you
don’t drop or break it.
4. The experiment requires the use of gases contained in
cylinders equipped with a regulating valve. The cylinder must
be securely strapped at all times. Consult your instructor on
proper use of a gas regulator.
Procedures:
Throughout this experiment you should record the uncertainty
(or notes so that you can determine it after class) for all
measurements including temperature, mass and pressure.
1. Read and Record the Barometric Pressure.
2. Record the starting pressure of the vacuum rack.
Part A: Average Molecular Weight of Air
1. Evacuate the Dumas bulb using the vacuum rack, close the
stopcock and remove the bulb.
2. Clean the outside surface of the bulb with acetone or
methanol. Handle bulb with gloves.
3. Weigh the sample bulb and the ballast bulb on the single pan
balance.
4. Open the stopcock, letting air, close the stopcock and
reweigh the bulb. Subtracting the weight of the bulb gives the
weight of air contained in the bulb.
5. Repeat Part A for a total of three runs.
Part B: Molecular Weight of CO2
1. Re-evacuate the Dumas bulb with the vacuum rack.
2. Turn on CO2 gas regulator, set to 15 psi.
3. Close Valve to Vacuum gauge
4. Use needle valve to introduce CO2 into the manifold until
atmospheric pressure is reached. (Watch manometer, levels

4. should be equal when at atmospheric pressure.) Adjust the gas
regulator on the CO2 tank as necessary when filling the vacuum
manifold.
5. Close regulator valve.
6. Close valve to bulb and weigh. Weigh the ballast bulb as
well.
7. Repeat steps 1-4 two more times.
Part C: Determine the volume of the Bulb
Do not do this part until you have completed parts A and B
1. Evacuate the Bulb on the vacuum rack.
2. Submerge opening to bulb in a beaker water. (Measure
temperature of water, too)
3. Open valve to bulb and suck in water.
4. Use a pipet to fill any remaining airspace.
5. Weigh bulb.
6. Use the density of water to obtain volume of bulb.
7. Record the pressure of the vacuum rack at the end of the lab
period.
Data Analysis
· Determine the buoyancy correction by comparing the mass of
the ballast bulb at different weighings. Apply the correction to
the sample bulb masses where appropriate and propagate error
as needed.
· Calculate the molecular weight of air using each of your mass
determinations. For one measurement, determine the uncertainty
in the molecular weight using the propagation of error method.
Compare to an established value for the molecular weight of air.
· Calculate the average molecular weight of air as well as the
standard error and confidence interval. Compare to an
established value.
· Calculate the molecular weight of CO2 using each of your

5. mass determinations. For one measurement, determine the
uncertainty in the molecular weight using the propagation of
error method. Compare to an established value for the molecular
weight of CO2.
· Calculate the average molecular weight of CO2 as well as the
standard error and confidence interval. Compare to an
established value.
Discussion
The discussion section of your report should:
· Restate the overall results of your experiment.
· Discuss your results in comparison to theoretical values for
the molecular weight of air and CO2.
· What was the effect of the residual pressure in your evacuated
bulb on the pressure? Did the fact that you didn’t have a perfect
vacuum limit the accuracy of your result? Do a calculation to
establish your answer.
· Discuss potential sources of error. What changes to the
experiment would you make to reduce this error.
References:
Physical Chemistry, David Ball, Thompson/Brooks-Cole, 2003.
Experimental Physical Chemistry 3rd Edition, Halpern and
McBane, W.H. Freeman, 2006.
Name:

6. Rubric for MW of a Gas Lab Report: Submit with Report
Section
Expectation
Points
Possible
Student
Evaluation
Instructor Evaluation
Abstract:
Summarizes Report
Includes a statement on experiment objective
3
Includes a statement on experiment procedure
3

7. Includes experimentally determined MW values
3
Includes comparison to established values
3
Introduction: Presents relevant background information
States main idea of experiment
3
Discusses theory relevant to experiment's objective and/or
methodology
5
Introduces relevant equations, defines variables
4

8. Written in student’s own words
4
Experimental: Procedure used so that experiment could be
reproduced.
Details of experiment provided in paragraph form. Make/Model
needed for commercial instrumentation.
4
Includes a diagram of apparatus used. Diagram is labeled, and
either drawn by hand or with computer drawing software. No
photos.
4
Results:
Presents data and results and explains calculations

9. Results presented in paragraph form, referring to tables/figures
where applicable
5
Includes at least one table summarizing MW air results (and
uncertainties)
8
Includes at least one table summarizing MW CO2 results (and
uncertainties)
8
Data analysis presented in paragraph form
5
Example calculations included.
5
Discussion:
Puts results in context, connecting to purpose of lab

10. Restates overall results of lab (MW of air and CO2)
3
Compares MW of air and CO2 to established values, taking into
account propagated uncertainty
6
Compares MW of air and CO2 to established values, taking into
account statistics (uses confidence interval)
6
Discusses the validity of the methodology for determining MW
8
Discusses the two approaches to reporting
uncertainty/comparing to established values
8
Accurately identifies one or more sources of error and discusses
how error may have impacted results
8
Discusses how the residual pressure in the evacuated bulb

11. impacts results and provides quantitative support for answer
8
Presentation
Report is neat and includes title page with appropriate
information (date, group member names, instructor name, etc.)
5
All figures/tables are properly labeled with numbers and
captions
5
All graphs and tables are properly labeled, easy to read and
accurately represent data
5
Writing is clear and concise, free from spelling/grammatical
errors and in proper tense/voice
5
Chemical terminology and symbolism is used accurately and
formatted properly.
5

12. Reference section is included, citing all sources used for
experiment (including source of literature values) and using the
ACS format
6
Other
Self-Assessment completed
5
Total
150

13. d =
PM
RT
d=
PM
RT
__MACOSX/untitled folder 4/._aaa.docx
untitled folder 4/part A & C.JPG
__MACOSX/untitled folder 4/._part A & C.JPG
untitled folder 4/part B.jpg
__MACOSX/untitled folder 4/._part B.jpg
untitled folder 4/Report 1.docx
Molecular Weight of an Ideal Gas by the Dumas Method
Lab Report #1
Abstract:
The main objective of the experiment is to determine the
molecular weight of air and CO2. The method that we used to
calculate the molecular weight is the Dumas Method. The
procedure is divided into three main parts, determining the
molecular weight of air, CO2 and the volume of the bulb which

14. helped in the calculations. The best value we got for the
molecular weight of air is 27.2 g/mol (+,- 0.1) which is close to
the experimental value 28.97 g/mol with percentage error (about
6.11%). The Percentage error we got in the molecular weight of
CO2 is 2.05%, since the theoretical value is 44.0 g/mol and the
best experimental one we got is 43.1 g/mol.
Introduction:
In this experiment, we measured pressure, volume and
temperature and weight of a gas sample in order to determine
the molecular weight of CO2 and air. The method that we used
is Dumas method which allows to calculate the molecular
weight. The theory states that we can determine the density of
the gas by calculating both mass and volume. Then, we will be
able to determine the molecular weight of air and CO2 using the
equation:
Where MW is molecular weight, R is gas constant, T is
temperature, P is pressure and M is the require variable which is
molar mass of the gas.
Experimental (Procedure):
The experiment included three main parts; average
molecular weight of air, molecular weight of CO2, and
determination the volume of the bulb. In the first part, we
evacuated the Dumas bulb, closed the stopcock and removed the
bulb. After that, we cleaned the outside surface and weighted
the sample bulb and ballast bulb on the balance. Then, we
reweighted the bulb after we let air inside it. We repeated this
part two more times.
Determination the molecular weight of CO2 is the second
part of the experiment. We re-evacuated the Dumas bulb with
the vacuum rack. We introduced CO2 into the manifold until
atmospheric pressure is reached. Then, we closed the valve to
bulb and weighted it with the ballast bulb too. We repeated the
three steps two more times.
In the third part, we determined the volume of bulb by

15. doing some steps; first, we evacuated the bulb, then we
submerged opening to bulb in a beaker water. After that, we
sucked in water by opening valve to bulb and used a pipet to fill
any remaining airspace. We weighted the bulb and used the
density of water to obtain volume of bulb. Calculations and data
analysis were done to determine the molecular weight of CO2
and air.
Results and Data Analysis:
Part A:Average Molecular Weight of Air
Pressure
Temperature
Density
Gas Constant
Water Mass
Volume
atm
K
g/ ml
L atm/ mol K
g
L
0.9899
298
0.997074
0.0821
271.6
0.2724
Table1: Shows the required values to calculate the molecular
weight of air.
Here the pressure is at the flask in the first part, water mass is
the difference between the two masses in the third part. We
used these values to determine the molecular weight as shown in
the following table.
Trial #

16. W1
W2
W2-W1
MW
Unit
g
g
g
g/mol
1
227.2 (+,- 0.1)
227.5 (+,- 0.1)
0.3 (+,- 0.1)
27.2 (+,- 0.1)
2
227.0 (+,- 0.1)
227.5 (+,- 0.1)
0.5 (+,- 0.1)
45.4 (+,- 0.1)
3
227.0 (+,- 0.1)
227.3 (+,- 0.1)
0.3 (+,- 0.1)
27.2 (+,- 0.1)
Table 2: shows the masses of the bulb and the molecular weight
of air in the three trials.
Where W1: weight of the bulb before letting air in; W2: weight
of the bulb after letting air introduce the bulb; MW is the
molecular weight of air.
We calculated the molecular weight by using two main steps.
The first step is to calculate the volume of the bulb by using the
equation: ; Since mass of water was calculated in Table 1
which is = 271.6 g; density of water was found in the density
table at the room temperature (25 °C);
0.997074 g/ml = ; so, volume = 272.4 mL = 0.2724 L
After that, we were able to calculate the molecular weight using

17. the formula; MW =
Trial 1: MW = = 27.2 (+,- 0.1)
Trial 2: MW = = 45.4 (+,- 0.1)
Trial 3: MW = = 27.2 (+,- 0.1)
In this part, we calculated the molecular weight of air using
three trials. Two trials were closed to the theoretical values
(will discussed in the discussion part). We used two steps and
two formulas to calculate the molecular weight of air.
Part B: Molecular Weight of CO2
Pressure
Temperature
Density
Gas Constant
Water Mass
Volume
Atm
K
g/ ml
L atm / mol K
g
L
2.29
298
0.997074
0.0821
271.6
0.2724
Table3: Shows the required values to calculate the molecular
weight of air.
The only value that is differ from Table1 is the pressure in the
bub which is almost doubled in this part. We used this values to
determine the molecular weight of CO2 as shown in the
following table.
Trial #
W1
W2

18. W2-W1
MW
Unit
g
g
g
g/mol
1
224.8 (+,- 0.1)
227.5 (+,- 0.1)
2.7 (+,- 0.1)
105.9 (+,- 0.1)
2
226.8 (+,- 0.1)
227.5 (+,- 0.1)
0.7 (+,- 0.1)
27.5 (+,- 0.1)
3
226.5 (+,- 0.1)
227.6 (+,- 0.1)
1.1 (+,- 0.1)
43.1 (+,- 0.1)
Table4: shows the masses of the bulb and the molecular weight
of air in the three trials.
Where W1: weight of the bulb before letting CO2 in; W2:
weight of the bulb after letting CO2 introduce the bulb.
Since the volume of the bulb was calculated in the first part. We
used only one step in order to calculate the molecular weight of
CO2. MW =
Trial 1: MW = = 105.9 (+,- 0.1)
Trial 2: MW = = 27.5 (+,- 0.1)
Trial 3: MW = = 43.1 (+,- 0.1)
In the second part, we got three values of the molecular weight
of CO2. The values were not close to each other. However, the
third trial is the closest one to the theoretical value. The
difference in the masses is the reason of getting three different

19. values.
Part C: Determine the volume of the Bulb
W1
W2
Twater
Troom
Pressure
Vacuum Rack
g
g
C
C
mmHg
mmHg
227.1 (+,- 0.1)
498.7 (+,-0.1)
24.5 (+,- 0.2)
25 (+,- 0.1)
29.6 (+,- 0.1)
49.0 (+,- 0.1)
Table 5: shows the weights of the bulb and the bulb when we
filled it with water.
W1: weight of the bulb; W2: weight of the bulb after letting
water introduce the bulb.
The values we got in this part were used to determine the
density and to calculate the volume of the bulb.
Discussion:
In this experiment, we were asked to calculate the molecular
weight of air and CO2. We did the experiment using two main
parts. We got three different molecular weights of each matter.
The closest molecular weight we got for air is 27.2 g/mol (+,-
0.1). By comparing this value to the theoretical one which is
about 28.97 g/mol.
% error = = = 6.11%
If we use the experimental value 27.3 as the uncertainty (+,-

20. 0.1), % error will be about 5.76% which is lower percentage
error than when we used 27.2. On the other hand, % error will
be higher if we used the value 27.1 (about 6.45%). The best
molecular weight we got for CO2 is about 43.1 (+,- 0.1) which
is close to the theoretical value (44.0 g/mol). % error = =
2.05%. The % error will be lower if we used the value 43.2
g/mol (about 1.82%) while it will be higher (about 2.27) if we
used the value 43.0 (-0.1).
Determining the molecular weight of air and CO2 by using the
Dumas method is an efficient and accurate one since we got at
least two close values to the theoretical one. However, we got
some wrong values of the molecular weights especially in
determining CO2 molecular weight. Some sources of error
might affect on these values, such as measurement errors. Also,
not cleaning the outside surface of the bulb may increase the
weight of it which affects the results. The residual pressure in
the bulb affected in the accuracy of the results since the
pressure will be higher and the might the mass too. If the mass
is higher than the actual one we will get higher molar mass, on
the other hand, if we get higher pressure we will get lower value
of molar mass.
Reference:
Physical Chemistry, David Ball, Thompson/Brooks-Cole, 2003.
Experimental Physical Chemistry 3rd Edition, Halpern and
McBane, W.H. Freeman, 2006.
4
__MACOSX/untitled folder 4/._Report 1.docx