Lab: Molar Mass of Butane
Purpose:
 To determine the molar mass of butane by using experimental data and to
compare it to the theoretically calculated molar mass.
Materials:
 Butane (C4H10) lighter
Safety Precautions:
 Butane is very flammable. NO FLAMES should be used in the room while this
experiment is in progress. This includes striking "strikers", lighting matches,
rolling the wheel on the lighters, etc.
 Butane gas is toxic, it will be released through the hood or outdoors.
 Wear safety goggles at all times during this lab!
Procedure:
1. Fill a gas-collecting tube completely with water, place your thumb or finger over the
open end and then invert the tube into a 400 mL beaker of water. If any air bubbles
entered the tube, try again!
2. Once you have the tube inverted and completely filled with water, secure it in place
with a clamp. The open end of the tube should be about in the middle of the beaker.
3. Mass the butane lighter to the nearest thousandth of a gram. Record the mass in your
data section. Be careful not to release any gas from the lighter.
4. Attach a section of plastic tubing to the butane lighter and work the free end into the
mouth of the inverted tube. Press the release button of the lighter making sure the
bubbles go into the tube. Collect between 35 - 40 mL of gas.
5. Remove the tubing from the lighter. Determine the lighter's new mass (to the nearest
thousandth of a gram) and record this in your data section. Return the lighter to your
instructor.
6. Draw a picture which shows the gas volume and label the following: eudiometer,
meniscus (xx.xx mL). See board for example. Title this drawing: Pgas < Patm
7. Transport your gas filled tube to a large graduated cylinder and lower it into the tube
until the water level inside the tube is even with the water level outside the tube. Read
the volume of the gas (to the nearest hundreth of a milliliter). Record this value in
your data section.
8. Draw a picture which shows the gas volume and label the following: eudiometer,
meniscus (xx.xx mL). See board for example. Title this drawing: Pgas = Patm
9. When you are done again put your finger over the open end of the tube, turn it upright
and take it to the hood to release the gas. Once in the hood, remove your thumb and
dump the water down the drain in the hood. This should evacuate the gas from the
tube.
10. Have someone from your group read the current room temperature and barometric
pressure. Record these values in your data section.
11. From Table A-7 (page 881) in the old Chemistry text, determine the "Water-Vapor
Pressure" exerted by the water vapor inside the tube. Record this in your data section.
Calculations:
Perform the following calculations:
a) Calculate the mass of butane removed from the lighter.
b) Determine the pressure exerted by just the butane in the tube. (Subtract the water
vapor pressure from the room (atmospheric) pressure reading.)
c) Convert the volume of butane from milliliters to liters.
d) Convert room temperature into kelvin.
e) Using the formula PV=nRT, determine the number of moles, n, of butane present.
f) Using your data from parts “a” and “e” calculate the experimental molar mass of
butane in grams per mole. (g/mol)
g) Using the chemical formula of butane, determine the theoretical molar mass of
butane in grams per mole.
h) Calculate your percent error. ((Experimental – Actual/ Actual) x 100)
Analysis:
1. Propane (C3H8) is another gas in the hydrocarbon family. If you were to go on a
camping trip and you needed to take along some portable fuel, would you get more
gas molecules with 1000 g of propane or 1000 g of butane? Explain your answer.
2. The two gas volumes indicated in your drawings are not the same. What is your
explanation for this?
3. A typical lighter contains 0.735 grams of butane in the liquid state. What volume does
this mass of butane occupy in the gaseous state at 25.0 C and 1.00 atm of pressure?
4. 0.817 grams of an unknown gas occupies 1.25 L at 27.0 C and a pressure of 765 mm
Hg. Is the gas most likely methane (CH4), ethane (C2H6), propane or butane? Show
work to support your answer.
5. Does a sample of methane gas diffuse faster or slower than a sample of butane gas?
Explain. (Use Graham’s Law of Diffusion)
Discussion:
If the gas release valve on the lighter is accidentally pressed between the time that the
lighter was first weighed and the time it is last weighed, would the calculated molar mass
of butane be too high or too low? Explain.