Lab 17 Molar Mass

Lab 17
Determining the Molar Mass of Butane
Grade Level Indicators
Construct, interpret and apply physical and conceptual models that represent or explain systems, objects, events or concepts.
Decide what degree of precision based on the data is adequate and round off the results of calculator operations to the proper number of
significant figures to reasonably reflect those of the inputs.
Develop oral and written presentations using clear language, accurate data, appropriate graphs, tables, maps and available technology.
Draw logical conclusions based on scientific knowledge and evidence from investigations.
Pre Lab Discussion
The ideal gas law, PV= nRT, (and n = m/M) provides us with a tool for the determination of molar mass of a gas from
physical measurements of mass, pressure, volume and temperature. Since the gas is collected over water, a correction is
made for the pressure caused by water vapor in the sample to determine the pressure of the dry gas. A sample of butane
gas will be obtained from a disposable cigarette lighter. The lighter is weighed both before and after the gas is withdrawn.
The volume, temperature and pressure of the gas are also measured. From these four quantities, the molar mass of the gas
will be calculated.
Pre Lab Question (Answer these questions on your own notebook paper!)
1. Write a statement of purpose for this lab in one sentence. (2 points)
2.
Using the equations, PV = nRT and n = m/M, develop an equation for determining molar mass (M) of a gas. (2 points)
3. What is the formula to find % error for an observed value? (1 point)
4. An unknown gas is in a sealed tank and the tank has a mass of 45.6751 g. The gas is then released and collected
over water at 22.0 °C into a eudiometer. The volume in the eudiometer is 48.92 mL. The tank has a mass of
45.5859 g after the gas has been released. Determine the molar mass if the barometric pressure when the gas
collected was 768.2 mmHg. Note: Follow the calculations for this lab to answer this question. SHOW ALL
WORK! (7 points)
5. What is the density of the unknown gas that was collected in the previous problem? (2 points)
6. Which gas (water or butane) has the most ideal behavior and why? (3 points)
7. Using a periodic table, determine the accepted molar mass of butane, C4H10. (2 points)
8. Make a data table to collect data in this lab. (3 points)
Materials
disposable lighter, eudiometer, analytical balance, 1-L beaker, electronic thermometer, barometer (online barometric pressure), paper towels
Procedures
o Determine the barometric pressure from an on line site. Record this Pressure.
o Immerse the lighter in water; remove. Dry the lighter thoroughly.
o Mass the thoroughly the dried lighter. Record this Mass.
o Fill a 1 liter beaker about three-quarters full with tap water.
o Using an electronic thermometer determine the temperature of the water in the beaker. Record this
Temperature.
o Fill the eudiometer all the way to the top with tap water.
o “Cap” with your index finger the eudiometer. Then invert and immerse under the water surface in a
large beaker near a sink.
o Hold the modified lighter under the opening of the eudiometer; press the release valve to release gas into
the eudiometer. Collect between 48-49 mL of the gas. (NOTE: Do not go over the 50 mL line if you do
you must start over!!)
o Remove the lighter; shake off excess water; dry with a paper towel. Mass the thoroughly dried lighter.
Record this Mass.
o Adjust the internal (eudiometer) and external water levels (beaker) until they are the same. Measure the
volume of gas in the eudiometer. Record this Volume. (Note: Remember all graduated instruments read
one more place than they are marked!)
Calculations (show all work on your own notebook paper!)
1.
2.
3.
4.
5.
6.
7.
8.
9.
Convert the barometric pressure from inHg to mmHg (Note 25.4 mm = 1 inch)
Determine the vapor pressure of the water vapor
Determine the pressure of the “dry” butane
Convert the volume to L
Convert the temperature in oC to K
Determine the mass of the gas collected.
Using your data determine the molar mass of the gas collected.
Determine the percent error of your experimentally determined molar mass.
Determine the density of butane at the lab conditions in which it was collected.
Conclusion (3 points for stating “your numbers” and 7 points for explaining your experimental error)
Write a conclusion by answering the purpose of this lab with the number you got (observed
value) for the molar mass of butane, the molar mass of butane you should of got (accepted
value) and the % error for your experimental number. Then give possible experimental errors
that explain why your % error was either too high or too low.
Lab 17
Determining the Molar Mass of Butane
Grading Protocol
Your Name : _______________________________________
Partner Name #1: _______________________________________
Partner Name #2: _______________________________________
Partner Name #3: _______________________________________
Partner Name #4: _______________________________________
Item
Name of Partners (shown above)
Grading Protocol stapled on top of the lab
Title & # of Experiment (shown on your own notebook paper)
Pre lab Question
Data table (With data and proper units)
Calculations
Conclusion (restate purpose with answer and sources of error)
TOTAL
Points/Out of
/1
/1
/1
/22
/5
/20
/10
/60