Molecular Mass of Butane

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Chemical Quantities and Calculations, Gases

Alternative titles: Molar Mass of Butane by Gas Collection

Summary

Butane from a lighter is collected in an inverted water-filled graduated cylinder. The mass of the lighter before and after gas release is used to find the gas mass. Using the ideal gas law, the number of moles of butane is determined and its molar mass is calculated.

Procedure

Fill a large container with water and invert a graduated cylinder full of water into it. Clamp the cylinder securely.
Weigh the cigarette lighter to obtain its initial mass.
Insert the lighter’s nozzle under the mouth of the inverted graduated cylinder and release butane gas so that it displaces the water, collecting a measurable volume of gas.
After collecting sufficient gas, remove the lighter and reweigh it to find the mass of butane released.
Record the volume of butane gas collected, the temperature of the water bath, and the atmospheric pressure.
Correct the collected gas volume for water vapor pressure.
Use the ideal gas law (PV = nRT) to calculate the moles of butane.
Determine the molar mass by dividing the mass of butane released by the calculated number of moles.

Links

Experiment 18 - The Molar Mass of Butane - Phil Reedy:

Determining the Molar Mass of Butane - Lab Hamster:

📄 Molar Mass of Butane - Flinn Scientific: https://www.flinnsci.com/api/library/Download/a2be5b406f58415b80fbcc3c02c9d932?srsltid=AfmBOop_1ugq9j-kmdhQpOIqqcOoaF4pfJRkr1_Q2bIzTOPJdCVcmX2u

Variations

Repeat the experiment with different collected volumes to improve accuracy.
Collect gases from other small lighters or gas cartridges to compare calculated molar masses.

Safety Precautions

Butane is highly flammable—keep flames and sparks away from the experiment.
Perform in a well-ventilated area to avoid buildup of butane vapors.
Wear safety goggles to protect from splashes or accidents.
Handle the lighter carefully; do not overheat or damage it.
Dispose of excess butane outdoors, not in a closed room.

Questions to Consider

Why must the gas volume be corrected for water vapor pressure? (Because the collected gas is a mixture of butane and water vapor.)
How does temperature affect the calculated number of moles of gas? (Higher temperatures increase gas volume at constant pressure, leading to different mole calculations.)
What assumptions are made when using the ideal gas law? (That the gas behaves ideally, with negligible molecular volume and no intermolecular forces.)
How would experimental errors such as gas leaks or inaccurate weighing affect the calculated molar mass? (They would lead to incorrect mass or volume measurements, skewing the molar mass result.)