AP Chemistry U05
Write Up Lab 1 The Molecular Mass of Butane
Skill set
“Collected Over Water” adjustments
Nonstandard conditions gas adjustment
Use of PV = (m/M) RT adjustment
Background: In this experiment, you will determine the molecular mass of butane, commonly found in inexpensive pocket lighters. You will release butane from the pocket lighter and allow it to displace a measured volume of water. From the displaced volume, you can easily calculate the molecular mass of butane gas.
Purpose: To determine the molecular mass of butane.
Materials: 250 ml Erlenmeyer flask glass plate
Trough butane lighter
Thermometer graduated cylinder
Procedure:
1. Fill a 250 ml Erlenmeyer flask completely full of water. Cover the top with a small glass cobalt plate without trapping any air in the top of the flask. (You may choose to use a 1000 ml graduate in lieu of the erlynmeyer flask at this point)
2. Fill a trough or other suitable container with water. Holding the glass plate in place with your finger, carefully turn the flask upside down and place it in the trough. BE SURE THAT NO AIR BUBBLES ENTER THE FLASK. Remove the glass plate.
3. Check out a lighter from your teacher. Drop it in the trough, take it out and wipe it dry with a paper towel. (you may choose to use the barreled lighter and not immerse in water but take care to only immerse the barrel when filling the flask or graduate)
4. Place the cleaned lighter on a balance pan and carefully determine the mass to the nearest hundredth of a gram. Record the mass in the data table.
5. Carefully hold the cleaned lighter under the water (note above). Make sure that the gas opening is beneath the mouth on the inverted flask, which you will lift slightly keeping the mouth of the flask under water at all times. Press the release lever of the lighter, being sure that all of the gas bubbles enter the flask.
6. Continue holding the lever down until you have collected approximately 200 ml or more of gas (more is better).
7. Remove the butane lighter, dry it with a paper towel and set it aside to use in step 10.
8. Do not remove the gas-filled flask from the water. Carefully raise or lower the flask until the level of water inside the flask is the same as the level of water outside the flask. Hold the flask at this level and carefully slip the glass plate under the flask. Hold the plate in place tightly and lift the flask out of the water and set it upright being careful not to spill any water.
CARRY YOUR FLASK WITH THE GLASS PLATE TO THE HOOD TO DO STEP 9
9. Remove the glass plate from the flask and with a graduated cylinder carefully fill the flask with water, keeping careful account of the volume of water needed to fill the flask completely to the top. The total volume of the water added with the graduated cylinder represents the volume of the butane as collected. Record the volume of the added water in the data table as the volume of gas collected.
10. Determine the mass of the butane lighter you used as you did before. Record the mass in the data table.
11. Measure and record the temperature of the water in the trough. Record the barametric pressure.
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Mass of lighter before experiment |
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Mass of lighter after experiment |
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Mass of butane |
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Volume of gas collected |
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Temperature (C) |
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Pressure (mm) (barometer reading) |
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Vapor pressure of water |
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Corrected gas pressure (butane pressure) |
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Calculations:
1. Determine the partial pressure of the butane gas. The gas
in the flask is not all from the butane lighter. There is also water pressure.
According to
Pbutane = Proom – Pwater
Pbutane=
2. The ideal gas equation gives the relationship between pressure, temperature, volume and the number of moles in a gas: PV = nRT where n is the number of moles. Replace n with that given below
mass (in grams) of gas or (m)
weight of one mole of gas (M)
mRT
to give : PV = M
where m = mass in grams and M = molecular weight. This equation may now
be arranged to solve for M or
M = mRT / PV
convert pressure in mm Hg to atm using the equivalence 1 atm = 760 mm Hg and
use this equation to calculate the weight of one mole of butane.
Use R = 0.0821 L ٠ atm/mole ٠ K. Remember to change the number of (ml) to liters also. Remember to show the work for your calculations in the writeup.
EXPERIMENTAL MOLECULAR WEIGHT = ______________ grams/mole
3. The formula for butane is C4H10. Calculate the weight of one mole of butane using this formula.
ACCEPTED WEIGHT OF ONE MOLE OF BUTANE = ___________grams/mole
4. Calculate your percentage error. Show work below.
% error = error X 100
accepted value
% ERROR = ____________%
Table
for Vapor Pressure of water
18°C 15.5
mm
19°C 16.5
mm
20°C 17.5
mm
21°C 18.6
mm
22°C 19.8
mm
23°C 21.0
mm