Determining the Molar Mass of a Vapor

Description

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• The term vapor is often used to describe the gaseous form of a substance which exists as a liquid or a solid under ordinary circumstances. Standard temperature is defined as 273 K; standard pressure is defined as 1 atmosphere, or 760 torr.
• The molar volume of a gas at STP is 22.4 L.
• Keep track of units, and use consistent units throughout.
• The vapor was produced in a boiling water bath, so the temperature at which the vapor filled this flask was 373 K. Please do not confuse the measured temperature with the defined standard temperature which is for gases collected at the freezing point of water instead of the boiling point of water.

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1. Set up a boiling water bath using a 400-mL beaker containing 250 mL of water.
2. Tightly cover the mouth of a 125-mL Erlenmeyer flask with a small square of aluminum foil. Use a straight pin to make a small hole in the foil cap.
3. Weigh the empty, capped flask.
4. Remove the foil cap. Place a 2-mL sample of the liquid to be studied into the flask and replace the foil.
5. Clamp the flask with a single buret clamp. Transfer the flask to the boiling water bath, immerse, and heat.
6. Note the liquid refluxing inside the flask.
7. Heat until liquid is no longer visible and no vapor condensate can be seen emerging from the pinhole. Continue heating 30 seconds beyond this time.
8. Remove the flask; set it on a hot pad; remove the clamp; and wait for the flask to cool to room temperature.
9. Dry the flask. Weigh the flask, cap, and condensed vapor.
10. Dispose of the contents of the flask according to instructions. Fill the flask with water. Pour the water into a 250-mL graduated cylinder, measure the volume, and record.
11. Measure and record the barometric pressure.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 078 Determining the Molar Mass of a Vapor

Mass of flask + cap =

Mass of flask + cap + condensate =

Volume of flask =

Barometric pressure =

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Name ___________________________ Class ________

Teacher__________________________

DoChem 078 Determining the Molar Mass of a Vapor

Watch the movie.

1. Identify the piece of equipment (balance, flask, barometer, graduated cylinder) that most limits the accuracy of this experiment. Justify your choice.
2. Calculate the molar mass of the vapor using the sample data.
   • Mass of flask + cap = 84.15 g
   • Mass of flask + cap + condensate = 84.77 g
   • Volume of flask = 137 mL
   • Barometric pressure = 757 torr

Closure Questions:

The substance used here is 1,1,1-trichloroethane, CCl₃CH₃.

1. Using the molar formula supplied by the teacher, calculate the actual molar mass of the unidentified substance.
2. Compare the true molecular mass of the substance to the experimentally determined value. Find your percent error.
3. Identify some likely sources of error in this experiment.
4. A gas has a density of 1.25 g/L at STP. Find its molar mass.
5. At STP, 10 liters of a gas has a mass of 13.4 g. Find the mass of 1 mole of this gas?

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Purpose

To determine the molar mass of a substance from measurements of the density of its vapor.

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(for 10 students working in pairs)

• 5 125-mL Erlenmeyer flask
• 5 400-mL beaker
• 5 250-mL graduated cylinder
• 5 support stand
• 5 burner, iron ring, wire gauze, or
• 5 hot plate
• 5 hot pad
• 5 straight pin
• 5 balance
• 5 single buret clamp
• 30 mL of unidentified liquid (1,1,1-trichloroethane)
• 1 roll of aluminum foil
• 5 thermometers
• 5 10-mL graduated cylinder

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• A volatile solvent with little intermolecular attraction works best. Chlorinated hydrocarbons work well but present toxicity problems. 1,1,1-trichloroethane is a compromise. Be absolutely certain to restrict the amount of 1,1,1-trichloroethane put out in the laboratory.

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Teacher preparation: 25 minutes

Class Time: 45-50 minutes

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1,1,1-trichloroethane is toxic.

Hot objects cause burns.

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• Control the amount of 1,1,1-trichloroethane put in the room to no more than 2.5 mL per 2 students. Provide adequate ventilation. Work in a hood if possible.
• Use caution when handling hot objects.

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Save unused 1,1,1-trichloroethane for use in later years.

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Presentation Question:

• Identify the piece of equipment (balance, flask, barometer, graduated cylinder) that most limits the accuracy of this experiment. Justify your choice.
  • The weighing error is much larger than are any of the other errors imposed by the apparatus used.

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(based on sample data for 1,1,1-trichloroethane):

Mass of flask + cap = 84.15 g

Mass of flask + cap + condensate = 84.77 g

Volume of flask = 137 mL

Barometric pressure = 757 torr

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(based on sample data for 1,1,1-trichloroethane)

Mass of the condensed vapor

= 84.77 g - 84.15 g = 0.62 g

Volume of the vapor at STP

Vstp = Vobs x (Pobs/Pstp) x (Tstp/Tobs)

Vstp= 137 mL x (757 torr /760 torr) x (273 K/373 K) = 101 mL

The molar mass of the gas sample:

= (0.62 g/ 0.101 L) x 22.4 L/mole = 1.4 x 10² g/mol

Percent error= ( | Experimental - Accepted value | / Accepted value) x 100%

= ( | 140 g/mol - 133.5 g/mol | / 133.5 g/mol) x 100% = 4.9%

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Closure Questions:

1. Using the molar formula supplied by the teacher, calculate the actual molar mass of the unidentified substance.
2. Compare the true molecular mass of the substance to the experimentally determined value. Find your percent error.
3. Identify some likely sources of error in this experiment.
4. A gas has a density of 1.25 g/L at STP. Find its molar mass.
5. At STP, 10 liters of a gas has a mass of 13.4 g. Find the mass of 1 mole of this gas?

Answers to Closure Questions:

1. The actual molecular mass of 1,1,1-trichloroethane is 133.5 g/mol.
2. Answers depend upon the substance. For 1,1,1-trichloro-ethane, the true molar mass is 133.5 g/mol, and this experiment led to a value of 140 g/mol. This value is 4.9 % too high.
3. Errors in mass determination are very important. An uncertainty of 0.01 g amounts to a 1.6 % change in the molar mass. When the liquid in the final flask has evaporated, the vapor pressure at room temperature has led to the displacement of some air. Since that air was in the flask at the outset, it should be weighed, too, so the resulting mass will be too low. Finally, the determination of the point, when no liquid is present, is difficult. Stopping too soon will result in large positive errors in the mass; stopping too late will result in small negative errors in the mass due to diffusion.
4. molar mass = density (STP) x molar volume (STP)

   = 1.25 g / L x 22.4 L/mol

   = 28.0 g/mol

5. molar mass = density (STP) x molar volume (STP)

   = (13.4 g/ 10 L) x 22.4 L/mol

   = 30.0 g/mol

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• molar mass
• density
• vapor
• volatile liquid
• pressure
• volume
• intermolecular attraction
• STP

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