Determining the Molecular Mass of a Gas
-Objectives
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Measure the volume, pressure, and temperature of a sample of wet butane gas, C 4H10(g).
Determine the pressure of dry butane gas by correcting for the partial pressure of water.
Calculate the molecular mass of butane and compare with accepted value.
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centigram balance
thermometer
pneumatic trough
250-mL graduated cylinder
Butane lighter
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-Materials-
-Procedures1.
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Put on your lab apron and safety goggles.
Fill a pneumatic trough with room temperature tap water. Record the water temperature in the data
table provided.
Fill a 250-mL graduated cylinder to the top with room temperature tap water. Invert, and displace air
bubbles.
Obtain a new butane lighter from your instructor. Take the mass of the lighter to the nearest 0.01
gram. Record your data.
CAUTION: At no time should you light the lighter. Be sure there are no flames in the lab.
As one partner holds the cylinder in place, the other should hold the butane lighter under the water,
just below the opening in the rack of the pneumatic trough. Without trying to light the butane, open
the trigger of the lighter and displace 200-250mL of water from the graduated cylinder.
Align the level of the water in the cylinder to the level of the water in the trough. Mark the level of
water. This will be the volume of your butane gas.
Remove the lighter from the water and dry it. When it is completely dry, take the mass of it, and
record the new mass. Record the volume of the butane gas on your data table.
Record the barometric pressure of the room in your data table.
Remove the graduated cylinder from the pneumatic trough, turning it upright as you do so. Take the
butane to an operating fume hood and pour the gas out.
-Data Analysis1.
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Subtract the final mass from the initial mass of the butane lighter. Record your answer.
Look up the vapor pressure of water at the temperature of the water in the pneumatic trough. Record
this value in kPa in your data table. Subtract this number from the room pressure reading.
The pressure of the collected gas will equal atmospheric pressure only if the levels of water inside the
graduated cylinder and in the pneumatic trough are the same. *Do the following only if levels are not
the same*
[Convert the height difference, if any, between these levels to kPa and subtract from barometric
pressure. (13.6 mm water = 1 torr & 760 torr = 101.3 kPa)
After making these corrections, calculate and record the partial pressure of dry butane]
Calculate your experimental value of molecular mass by rearranging the ideal gas law expression,
PV=nRT, in this manner: n = PV/RT. P is equal to the partial pressure of the dry butane in kPa; V is
equal to the volume of gas collected in dm3; n is equal to the number of moles of butane; T is the
absolute temperature in degrees Kelvin.; R value is 0.0821 L•atm/mol•K.
Calculate the % error for your experimental value of molecular mass. This is done by first finding the
difference between the accepted value and the experimental value. Then the difference is divided by
the accepted value and multiplied by 100.
-Data / Observations Initial mass of butane lighter
Final mass of butane lighter
Barometric Pressure of
room
____g
____g
____kPa
____mm Hg
Temperature of water
____οC
Volume of gas collected
____mL
-Analysis Results Mass difference of butane
lighter
Vapor pressure of water
at room temperature
Partial pressure of dry
butane gas
Your value of molecular
mass
Accepted value of molecular
mass
Percent error in your value of
molecular mass
-Analysis
1.
2.
____g
____kPa
____kPa
____g/mol
____g/mol
____%
/ Results
If your percent error is large, what factors do you think account for this error?
Would the value of your molecular mass go up or down if you had not corrected the gas for partial
pressure of water? Why?