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Molar Mass of Butane (0 Favorites)

LAB in Gas Laws, Pressure, Measurements, Molar Mass, Ideal Gas, Partial Pressure, Error Analysis. Last updated December 23, 2020.


Summary

In this lab, students will experimentally determine the molar mass of butane using Dalton’s law and the ideal gas law. They will also calculate the percent error and explain possible sources of error.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to the following scientific and engineering practices:

  • Scientific and Engineering Practices:
    • Using Mathematics and Computational Thinking
    • Analyzing and Interpreting Data

Objectives

By the end of this lab, students should be able to:

  • Use Dalton’s law and the ideal gas law to find the number of moles of a collected sample of butane
  • Determine the molar mass of butane from a disposable lighter
  • Calculate the percent error of an experiment and analyze where error may have occurred

Chemistry Topics

This lab supports students’ understanding of:

  • Gas laws
  • Molar mass
  • Percent error

Time

Teacher Preparation: 30 minutes
Lesson: 60 minutes

Materials

Per group:

  • Balance with precision to at least 0.01 gram
  • Butane lighter
  • Graduated cylinder, 100 mL
  • Thermometer
  • Large water bucket (room temperature)
  • Barometer – Alternate: can use internet to find pressure for the area
  • Paper towels

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
  • Keep butane away from open flames as it is highly flammable.
  • When students release the collected butane at the end of the lab, be sure that they do so in a fume hood or outside.

Teacher Notes

  • This lab is designed to be performed after instruction on molar mass, gas laws, and percent error.
  • Students can be in groups of 2 or 3.
  • In order to speed up gas collection, you may want to increase the flow on the disposable lighters. This video shows you how to do that. At the same time, you will also want to remove the striker/flint from the lighters so that they will not light, as the flames would be very large and potentially hot enough to melt the plastic of the lighter and ignite the remaining butane once you have adjusted the flow. Watch this video for some guidance on removing the flint.
  • If a barometer is not available, look up atmospheric pressure for the area.
  • Most beakers will not have a wide enough opening for students to put their hands with the lighter under the graduated cylinder in the water, so be sure that you get a bucket or other container that is large enough. It might be helpful to test the set up yourself first.
  • It may be helpful to put an upside-down funnel over the lighter with the thin end inserted into the opening of the graduated cylinder to direct the gas into the cylinder more precisely.
  • Because the density of gases are so low, the masses collected will be very small. Thus, if students’ measurements are off even by a small amount, the percent error could be quite large. If you have the appropriate equipment, you could use larger graduated cylinders and collect larger amounts of gas to minimize this effect. The balances you use should report at least 2 decimal places, but use the most precise balances you have.
  • The butane lighters should be immersed in the water before the initial mass is taken and then dried with a paper towel.  When obtaining final mass, dry the lighter with paper towels before obtaining the mass.  The biggest source of error is the mass of the “dry” butane lighter before and after collecting the gas.  Immersing in water before the initial mass will take into account any trapped water after submersion.
    • Alternatively, students could take the initial mass of a dry lighter, complete the experiment, and allow the lighters to dry overnight before taking the final mass of the lighter. For this to work, you would need to have one lighter for each lab group that they would label with their names and you couldn’t reuse the same lighters in other classes throughout the day.
  • Collected butane gas and butane gas left over in the lighter should be released in a fume hood or outdoors.
  • If your students are more advanced, a similar AACT resource, Determination of the Molar Mass of Butane, has students design the procedures themselves that would allow them to determine the molar mass of butane, rather than providing procedures for them. This would be more appropriate for second year/more advanced chemistry classes, such as AP, IB, or dual credit Chemistry.

For the Student

Lesson

Background

Butane is an organic compound with the formula C4H10. Very pure forms of butane can be used as refrigerants, for instance in household refrigerators and freezers. Butane is also used as lighter fuel for a common lighter. Butane is a highly flammable, colorless, easily liquefied gas that quickly vaporizes at room temperature and pressure.

In this lab, you will collect a measurable volume of butane gas using water displacement. Using Dalton’s law and the Ideal gas law, the number of moles of butane gas and then the molar mass of butane can be calculated. Your experimental value for molar mass will be compared to the known molar mass of butane and percent error calculated.

Dalton’s Law:      Ptotal = Pbutane  +  Pwater vapor

Ideal Gas Law:    PV = nRT

Percent error:    |accepted – experimental| x 100%
                                           accepted

Prelab Questions

  1. A sample of nitrogen is collected over water at 27 0C. What is the pressure of the nitrogen if the total pressure is 760. mm Hg?
  2. A sample of an unknown gas has a mass of 0.25 g, pressure of 770. mm Hg, and a volume of 0.10 L. If the temperature is 25 0C, what is the molar mass of the gas?
  3. In step one of the procedures, you are told to submerge the lighter in water before taking its initial mass. Why do you think you are asked to do this?

Objective

Experimentally determine the molar mass of butane.

Materials

  • Balance with precision to at least 0.01 gram
  • Butane lighter
  • Graduated cylinder, 100 mL
  • Thermometer
  • Large water bucket (room temperature)
  • Barometer – Alternate: can use internet to find pressure for the area
  • Paper towels

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • Keep butane away from open flames as it is highly flammable.
  • When you release the collected butane at the end of the lab, be sure that you do so in a fume hood or outside.

Procedure

  1. Place the lighter under water. Remove the lighter, shake it off and dry the excess water with a paper towel. Using the balance, determine the mass of the lighter and record. 
  2. Fill water bucket two-thirds full with room-temperature water.
  3. Fill a 100 ml graduated cylinder to the absolute top with water. Place your hand over the top of the graduated cylinder and invert into the bucket underneath the water. Make sure the graduated cylinder has no air in it once it has been inverted.  See Figure 1.
  4. One group member should hold the butane lighter under the water while another holds the graduated cylinder above it. The student holding the lighter should depress the plunger on the lighter until at least 90 mL of butane is collected in the graduated cylinder above it. Do not let any butane escape outside of the graduated cylinder. See Figure 1.
  5. Place the thermometer in the bucket and record the temperature in 0C.
  6. Carefully lift the graduated cylinder until the level of butane is level with the water level in the bucket. While maintaining equal water levels, read the exact volume of the butane gas to the nearest 0.1 mL and record. See Figure 2.
  7. While keeping it upside down, remove the graduated cylinder with your hand covering the opening and take it to the fume hood or outside to release the collected butane gas safely.
  8. Shake off the butane lighter and dry the outside with a paper towel. DO NOT depress the plunger on the lighter! Record the mass of the lighter.
  9. Record the barometric pressure using a barometer or look up the current pressure for the area on the internet.

Data

Measurement Value
Initial mass of the lighter
Final mass of the lighter
Temperature of the water
Volume of butane in the graduated cylinder
Atmospheric Pressure

Calculations

Show all work for each of the following calculations.

  1. Determine the mass of butane collected.
  2. Determine the vapor pressure of water using the chart below.
Temperature (°C) Vapor Pressure (mm Hg) Temperature (°C) Vapor Pressure (mm Hg)
15 12.788 23 21.068
16 13.634 24 22.377
17 14.530 25 23.756
18 15.477 26 25.209
19 16.577 27 26.739
20 17.535 28 28.349
21 18.650 29 30.043
22 19.827 30 31.824

Temperature of water: ________     Vapor Pressure of water: __________

  1. Determine the pressure of the butane gas collected by using Dalton’s law. (Note that Ptotal = Patm)
  2. Using the ideal gas law (PV = nRT), determine the number of moles of butane gas collected. Make sure you convert your measurements to the appropriate units for the ideal gas constant, R, if they do not match up.
  3. Determine the molar mass of butane using your experimental data.
  4. Determine the accepted value for the molar mass of butane, C4H10, from the molar masses on the periodic table, and use this to calculate the percent error of your experimental results.

Analysis

  1. Butane gas doesn’t dissolve very well in water. Why is this necessary to perform this experiment?
  2. How could this procedure be used to identify an unknown gas?
  3. Identify at least 2 possible sources of error and how they could have affected your results.

Conclusion

Write a paragraph to explain how you determined the molar mass of butane. Explain possible sources of error and how you could adjust your technique or the materials or procedures to minimize those sources of error if you were to repeat the experiment.